Keflex belongs to a class of potent antibiotics called cephalosporins. It is one of the first of these to become available generically and is now often prescribed as cephalexin.

This drug works against a wide variety of germs, including those that cause infections of the skin, lungs, throat, prostate, urinary tract, bones, and ears.

Cephalosporins are broad spectrum medicines that were originally discovered in one of the world’s most unlikely locations.

A fungus found close to a sewer outlet along the coast of Sardinia turned out to cure a number of nasty infections. From this chance observation many extraordinary antibiotics have been developed.

Side Effects and Interactions

Side effects from cephalosporin-type antibiotics are generally mild. Nevertheless, cephalexin can cause a range of digestive tract disorders.

Indigestion, stomach pain, nausea, vomiting and diarrhea have been reported. If diarrhea becomes severe contact your physician, as it may be a warning of drug-induced colitis.

Other infrequent reactions to be aware of include rash, itching, vaginitis, headache, confusion, joint pain, fatigue and dizziness.

Cephalexin may affect laboratory test results. Liver enzymes may become elevated and false-positive results may show up during certain diabetes tests. Make sure that the laboratory personnel are aware you are taking cephalexin if you have blood drawn.

And remember to report any symptoms or suspected side effects of cephalexin to your physician promptly.

Special Precautions

If you are allergic to penicillin-type antibiotics, alert your physician immediately. Some people who are sensitive to penicillin may also react to cephalexin.

Symptoms such as breathing difficulty, wheezing, sneezing, hives, itching, and skin rash require immediate emergency treatment.

Life-threatening anaphylactic shock may produce an inability to breathe and cardiovascular collapse and can occur within minutes of exposure.

People with kidney problems should receive cephalexin only under careful medical supervision as the dosage will most likely have to be modified to prevent toxicity.

Taking the Medicine

Although this antibiotic is absorbed more efficiently when it is taken on an empty stomach, the pills can be swallowed with food, especially if they upset your stomach.

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  1. Suzy

    A week ago I had bladder surgery and afterwards was placed on 500mg Cephalexin every 8 hours. A couple of days ago I noticed a rash on my arm. I took a Benadryl and the rash was gone that evening. Tonight my back felt itchy and after looking in a mirror, a rash was all over my back. I took another Benadryl and will be calling my doctor in the morning about this.

  2. Turtleman
    North Carolina

    I’m having to take 500 mg. of keflex 4 times a day and 500 mg of ceepro 2 times a day. How much probiotics should I be taking to help put good bacteria back into my system?

  3. Michelle
    Rochester, NY

    I got Cellulitis in my leg from a spider bite and was given Cephalexin. After completing the 10 day supply, I kept telling my boyfriend I still wasn’t feeling good, dizziness, nausea (but no vomiting), fatigue, headaches, itching and rash. Yeah me, now a reaction to the Cephalexin. However, at the same time I got the spider bite diagnosis, I also was diagnosed with s severe case of poison ivy and was given prednisone to help with that, so I can’t have that again so soon… my question is, how long do these effects last? I am so tired of itching!!!

  4. Staci

    My daughter was taking Cephalexin for an infection on her ankle. She had a sore that she picked at until it became infected and turned into cellulitis and possibly MRSA. Anyway, she was given two antibiotics, one of which was cephalexin. She finished one of them, but still had two more days worth of Cephalexin left when she developed a red itchy rash all over body and swollen lips. It started on her arms and chest, and looked like a sunburn, but then quickly worsened and spread all over her entire body. Plus her lips are swollen.

    No fever, no nausea, no vomiting, no body aches, but she does have a head ache and is super tired and irritable, or maybe that is just her. Anyway, my concern is whether or not this will develop into something more serious, or will it just go away eventually since I have taken her off the Cephalexin. I don’t want to take her into the doctor just to be told to take her off the cephalexin and keep an eye on her. I gave her benadryl which didn’t seem to make a difference.

    Any advice? My daughter is 15 by the way. Also, if she is allergic to the Keflex family, will she be allergic to other types of antibiotics?

  5. Rob
    CA, USA

    I’m a 21 year old male, and was prescribed 500mg of Cephalexin twice a day for Pharyngitis which is very similar to strep-throat, but with more severe swelling. Anyways, it helped the swelling go down but I felt very nauseous and dizzy in general on Cephalexin. The next day, I also had very bad stomach pains – as if there were a bubble in my lower abdomen. Suddenly, I was waking up every morning around 4am or 5am with severe stomach aches and felt like vomiting. The discomfort in my stomach is still there several days AFTER I stopped taking the medication and I’m still feeling pretty light-headed even now.

    I seriously regret taking this antibiotic and will be visiting a doctor again soon to find out if these side effects are. I recommend avoiding this medication at all costs – as for me, it’s a world of pain.

    I’m one of those people who are allergic to most antibiotics.

    • elizabeth
      las vegas,nv

      Hi rob,
      Im going through the same ordeal.. ive stopped taking the meds, its only been 1 day so far.. i hate these things. My question is how many days post abx were you lightheaded for? Cuz i cant deal

  6. Kim
    Ansonia, CT

    Your Doctors should’ve all told you that after you take the course of antibiotics for the infection you have that you should replenish your system with good PROBIOTICS. I am on Keflex right now for a UTI however the ER doctor agreed with me on taking the probiotics I had already ordered after seeing the urgent care doctor about not feeling well.

    I had been given Cymbalta years back and that messed up my digestive track. Taking probiotics has helped immensely with that. I also was not absorbing all the vitamins I tried to take daily. I found a patch for that now. So far so good. Ask your doctors about taking probiotics after completing antibiotics. And ask about a multivitamin patch.

  7. Kayla

    I took keflex recently and have had pink (bloody) vaginal discharge for 5 days. It started a few days after I stopped taking the keflex.

  8. kj
    Lawton Ok

    I too have been taking Cephalexin for 5 days now and I too am experiencing problems with the vaginal area it’s very uncomfortable does this mean I’m allergic and how do I seek relief? And also my left side is still in pain does that mean the meds aren’t working??

    • Staci

      You probably have a yeast infection and should take Monistat or something. The one day treatment or 3 day treatments don’t really work. Monistat does burn when you take it so if you get that make sure to get the kind that has the cream which helps to stop the burning. Or you can get a pill from your doctor that takes the yeast infection away. I think the pill’s name starts with a D but I can’t remember. Good luck.

      • CLee

        The name of the pill is “Diflucan” for yeast infection after antibiotic therapy!

  9. Schebania P.

    I was on keflex 4 days ago, and I have experienced nausea, constipation and overall fatigue. I am glad I checked this site cause I have been feeling so bad and could not explain why. At least now I know what was causing all these symptoms… I will definitely up my water intake to get rid of any left over drug from my system.

  10. Rose
    United States

    I was placed on an IV of Rocephin for 7 days with a combination of Bactrim for an infected bursitis on my elbow. I was switched from Rocephin to Keflex after 7 days due to a change of doctor. I was also prescribed Tamiflu b/c I was showing flu like symptoms. I was kept on the Bactrim even though it was giving me continuos diarrhea. After the first day on Keflex (4 doses in 24 hrs) I woke up with hives and a high fever. I immediately stopped the Keflex and Tamiflu and took Tagamet and Benadryl. The hives spread down my torso onto my arms and legs, the fever and body pain continued into a burning aching. I was hospitalized 2 days later for extreme dehydration and hyponatremia.

    After 2 days of getting fluid drip, morphine and oxycodone in the hospital I was released. I immediately began suffering body and joint aches and pains to the point I could barely walk as well as constipation. I was the prescribed prednisone which only seemed to help very little after 7 days of being on it. Since I have stopped prednisone as well as all other antibiotics I have continues joint and body aches as well as an all over body itching that has no rash or sign of discoloration. My primary care physician as well as my rheumatologist have applied the wait and see method, but I am starting to go insane with the pain and itching.

    Has anyone experienced those symptoms after being on any of those medications and for how long after you stopped taking them?

    • Stacie

      I to suffered from joint pain and really bad itching after taking cephalexin, and the sad thing about it all is that I stopped taking the medicine in January and it is now April and I’m still experiencing these bad pains and itching. And I was never bothered with joint pain or itching prior to taking it. I go see my Rheumatologist for the first time tomorrow because my husband finally told me enough is enough and I need to be checked out because it’s really bad.

    • m

      Constipation and joint/body aches are likely from the morphine and oxycodone drip and/or drip “withdrawal.” They could likely take a while to go away, and there’s really nothing you can do. I’d try to not take anything for a while.

  11. Sunny

    I started taking the generic version of this about 24 hours ago and have now thrown up twice and feel nauseous and tired. I was given this for an infection in my tooth and have taken it before and never had any issues. I called my dentist and was told nausea and vomiting are not a side effect and to continue to take this medicine.

  12. CC
    PSL, Florida

    My husband was just told he has strep throat today and was given cephalexin to treat it. We arrived home at noon today he took one w/ lunch. He has had a severe headache and raging fever and nausea all afternoon. The Dr. suggested to take three today, so he took another one w/ some soup at 5. He feels worse now then he has this past week, and he has been feeling really bad. I read the side effects and all his symptoms were right there. I am not giving him another cephalexin and will be calling the Dr. for something different tomorrow. I guess research is the best advice after 5 when the Dr. is gone for the day and Thank Goodness for sites like these to inform everyone. I hope more people will stop taking this antibiotic at the first sign of side effects. They are listed for a reason, if you experience them even one, stop taking the drug, it is just not worth the suffering in my opinion.

    I hope this helps someone else out there trying to decide what in the world in happening to them all because they took an antibiotic.

    • Nicole

      Always ask a pharmacist they know more about the drugs then the doctor

  13. Kristen

    I have been on this antibiotic for a UTI. Have not had any bad side effects. It seems to be working well for me. Eating yogurt everyday was recommended by pharmacist.

  14. Faith

    My medical doctor prescribed 500 mg 4 x a day for kidney infection for 14 days. I actually shocked at how sick and nausea, my stomach pain is unreal, all my organs were checked for inflammation just to be on the safe side, all looks fine. I called my naturopathic for second opinion to take a closer look and he said, antibiotics is what you have to take for kidney infection. I decided to take it but if I get any sicker I’m requesting an antibiotic IV because I’m don’t want to suffer anymore stomach pain. I have another plan in place if i cannot antibiotics that I know. Cures without all the side effects.

  15. Danny

    I took it only for seven days and passed out and was taken to the hospital near death.

  16. cristina

    This drug has made me psychotic as did norfloxacillin many years ago. Came down 2 months ago with such severe pain on top of my left hip that I couldn’t stand and was nearly passing out. Hospital put me on an anti-biotic and rehydrating drips. Tests on urine showed UTI and ultrasound showed absence of passing a kidney stone.

    Was put on course of flufloxacillin. Doctor back home tested urine which still showed nitrates (sign of infection) and put me on a course of keflex. Have finished course 10 days ago and still got UTI and the symptoms mentioned in above comments (nightmares, feeling of impending doom, getting stuck in negative thought patterns, insomnia but no digestive problems) have not faded.

    Have refused to take a take a fourth course of antibiotics so am going to exist on a diet of pure cranberry juice, steamed vegetables with live yoghurt and probiotic pills for 6 days then have my urine tested again. Have not slept for more than 14 days ago despite taking mild medication to help me. Friends say I look like death warmed up. How long will the side effects subside? I will tell you if my home grown treatment works.

  17. Nicki
    Portland Oregon

    I took this for my cyst everything was going smoothly. Until I got major headaches and the light bothered me. Then I would fall into a deep sleep and wake up with confusion feeling numb. So I went back to the doctor and she said I’m fine and to take Advil.

    OKAY I did and then I got brain fog and woke up again not knowing where I was so my sisters rushed me to the hospital. 4 hours later my doctor tells me I’m fine and to finish pills 500 mg and to take 600 mg of ibuprofen :( finally I asked him about my symptoms, and he told me that the pill only causes diarrhea or stomach aches and I may be going to sleep with a migraine, actually, he said he doesn’t know. I told him I am not going to finish my medicine and I will find a new doctor. But the new doctor prescribed me Bactrim as well. Why oh why do they stick pills down clients throats – I’ll never understand.

  18. Ron W

    It’s not the drugs its the doctors with ADD. Sent home from hospital with cloudy urine and negative bacteria. Urine progresses to a light cider mill apple juice cloudiness. In hospital (different one) UTI positive, injection of antibiotic, and sent home on keflex 500 four times a day.

    Second doctor told me I was taking it wrong (500 every three hours) should take two five hundreds every twelve hours and just started. Have ileostomy (from UC). with chronic obstruction, Since keflex unbelievable weakness on top of my chronic fatigue, bloating, pain, aches, numbness of left face. MRI taken of head normal but have trigeminal “neuropathy” nor neurolga (means pain). I am told its possibly from either diabetes, infection, or tumor after MRI. Appointment with specialist soon,, Meanwhile mostly left side aches in shoulder, stoma area (hernias), knees, wrists, finger joints, abdominal pain around stoma incessant.

    Second doctor wants me to continue and extend use of keflex as I am 69, I have stage 3 CKD and renal doc says it will not affect my kidney. I am miserable and feel I have a alien in my belly which is full of hernias, taking miralax, disgestive enzymes. Other conditions HBP. Osteoporosis, Sciatica, and spinal degeneration.

    Though diabetic its under good control but I am peeing often (distended bladder). Temperature ranges from 97 to 99. Very achy and feel I have an infection. Urine now clear and yellow. Would like to hear from ileostmate who may have similar problems. I am hoping to have surgery soon to correct chronic pain from adhesions and partial obstruction of ileum though I am told this will be risky. Cannot take the cramps from blockage much more so I am willing to take the risk.

    Keflex seems to have added gasoline to the fire but I am told UTIs can be very dangerous and can lead to kidney infections, failure as well as bone infections. Scared as I have a history of sepsis from UTI as well as ventral incision MRSA.

  19. fenella

    Started taking this drug pre-surgery and have continued. Can’t tolerate it enough to be able to finish it. The chest and abdomen pain is constant, burning and I feel so sick but can’t vomit. It’s really horrible. Also have a chest infection and the coughing brings the nausea up. Never, ever consuming this drug again.

  20. jd

    I am also glad for finding this site. I’m having the same vaginal issues and it began the day before I had to take the last pill. Irritation is unbearable and swelling smh… How can I get comfort?

  21. ART

    So I was prescribed this medicine when I had a infection in one of my testicles, after a couple of days later it went away, and everything is fine, then one day I woke up with a rash near my penis and I still have it.
    I’ve put cream on it (mostly eczema cream and neosporin ) and yet when I wake up the next day, it’s still all flakey and rash like. Now after 6 days of being off the medicine, I wake up at 2, 3, 4 o’ clock in the morning feeling sick and feeling like I have to throw up, but never do. Couple this be depression? I’ve been a little depressed lately over a health issue. Could it be Allergies?
    I’ve also been constipated, my poop has been coming out in balls. If this is all from the medicine, never taking it again.

  22. LP

    Steroids are contraindicated when one has an infection!

  23. Barbara C

    Wish I had not waited so long to check this site. This is all very scary for me. Just had breast cancer day surgery last Tuesday and was given a prescription for CEPHALLEXIN; 500mg 3 times daily. Just 2 days left. I felt great after surgery (Lumpectomy plus 2 lymph nodes)except sore throat and congestion from surgery.
    But these last few days my chest feels congested, I feel drained of energy. Despite feeling worse instead of better I continued to take the meds because I was afraid of pneumonia or infection of some kind. I am not taking any more. I am supposed to be walking 3 times a day but my heavy feeling chest, general all over achy painy feeling, and lack of energy is making it difficult. Of course the TERRIBLE POLLUTED AIR is no help either. My back has been getting very itchy now, too.
    (And I thought since the pathology report said no cancer in lymph nodes or margins that everything was okay. But the radiology oncologist pointed out to me that I had INVASIVE DUCTILE CARCINOMA that is HORMONE DEPENDENT, that I had CANCER. The cancer had grown through the duct into the breast. He said he might be able to cut the treatments down to four but first I needed to see the medical oncologist to probably be put on a hormone blocker type med and for me to stop wearing my patch that I was only feeding the cancer! If I did that, I could possibly opt out of radiation treatments since I am 75.)

  24. DS

    So, back in the day as a child I had an odd reaction to an old school cephalosporin where my stomach got really hard & painful & I was unable to eat because food would come right back up. As an adult I have taken Keflex a couple of times with no issues. This past Friday my PCP put me on Keflex for an infected cyst that is unable to be removed until the infection is gone. A short time after taking my 1st dose I developed an uncomfortable feeling behind my sternum, almost as if something was caught in my esophagus or like my esophagus was stuck in a muscle spasm. I have had this feeling ever since my 1st dose on Friday and it is only gone when I wake up in the morning but returns after my morning dose. The feeling goes away for a bit when I eat or if I take a deep breath and hold it for a while and slowly exhale. Some times the pressure is so bad it feels like I am going to gag or throw up. It seems like a rather odd reaction to a medication. Has anyone else experienced this with a cephalosporin? What are the odds it’s something totally unrelated to me taking this antibiotic?

    • Debra-L.
      Forest Ranch, CA

      I am currently taking Cephalexin (generic for Keflex) after surgery for nodule on thyroid. I too am experiencing the pain behind the sternum. If feels to me like a big bubble is sitting there. I feel like a big burp would help but it doesn’t. I have experienced this once before but I don’t remember if it coincided with taking antibiotics. I am also taking Prednisone for inflammation. I avoid medication and rarely have to be on antibiotics.

  25. TH

    Thank God I found this site! I thought I was losing my mind when 2 days after stopping Keflex I began to experience vulva itching and swelling. I knew it wasn’t an STD as I’ve been married forever and had just had a normal pelvic exam only a few weeks ago.
    At first I thought maybe it could have been a reaction from the Florastor probiotic I’m taking to prevent C Diff. But now I’m realizing from all the comments here that it’s the Keflex! I will never take this antibiotic again.
    To help with the itch and a “dry” patch on the skin I began rubbing Manuka Honey 18+ on it. It’s a bit pricey, but I use that stuff for everything…it’s awesome. It’s been 3 days and it’s soooo much better, I can at least function now.

  26. td

    I have taken Keflex many times. It ALWAYS gives me thrush and a yeast infection. It usually but not always gives me diarrhea. I just take an Imodium with every dose. This time I was taking it for phlebitis. I got exhausted where I just didn’t want to move. Taking with probiotics helps.

  27. ht

    I took this for cystitis but I had a terrible reaction to the tablets last night. I felt the same feeling just a little the night before but didn’t think much of it and contd the tablets yesterday.
    Well, I paid for it. Muscle spasms, on a super high, body wouldn’t relax, diarrhoea, nausea (after taking every tablet), weakness, brain fog and no sleep til 4am for 20 mins when the reaction lessened a bit. Scary as and feeling awful today, to be expected, and I won’t be taking the last 3 tablets. I went with this brand because I had no reaction to it 8 years ago. I’ve had joint pain too, and stool problems (constipation) which ended up in diarrhoea last night – didn’t realise it could be from this.

  28. Diane

    Hello, I found this site after looking for a relationship between Keflex and herpes. I took Keflex and within three days had a huge cold sore break out on my mouth. This sore had been dormant for about 15 years! I was sure there was some cause and effect from the drug and now it seems accurate.

  29. BKO

    Been taking generic cephalexin for the past four days to prevent possible infection after having knee surgery. I noticed the last few days that I have been feeling light headed, dizzy and nauseous about a half hour or so after taking my meds. Today I decided to take each med independently. I started out first today with the cephalexin and sure enough about a half hour later I started to feel dizzy and nauseous. Additionally my back has been itching uncontrollably over the last couple days. I am fairly certain that it is the generic keflex (cephalexin). The other meds I have taken before without any side effects. Though I rarely take medications this is the first time I have ever had a noticeable negative reaction.

    • Bill C

      I have been off this drug for 7 days. I feel miserable. I have all the same side effects. Have you felt relief? If so, how long did it take?


  30. KE

    I started a course of this drug on Mon @ one capsule x 4/day to address a possible infection at my breast. (Total consumed as of 2/21/14 = 14) By day 2, I noticed less burning and pain at said breast, as well as a decrease with swelling, HOWEVER, I began experiencing chest, shoulder and lower back pain (muscle versus joint or so I think) almost at once. My breathing has been shallow at times, to the point of having to sigh in order to get a full inhlation, although this has been intermittent.
    My vagina has been tingling as if a yeast infection is about to begin. I have had a sore throat and a headache. I have also had typical intestinal issues (gas, bloating and increased stools). I called my PCP and spoke with him via a RN and PA… uh, yes. Anyway, he said I should stop taking the med at once and monitor my course over the weekend. All I know is that my body is out of sorts and that it feels quite foreign at this point. This drug and the quick fire presentation that has resulted in response to my taking it are a serious concern for me. This med is a definite no go as far as I am concerned. I am posting so that others might be helped.

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