Keflex (generic name: cephalexin) 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 of Cephalexin (Keflex)

Side effects from cephalosporin-type antibiotics are generally mild. Nevertheless, it 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. 'oldie&how'

    Have had Keflex before without any problem but Alprim didn’t work for Cystitis so am on
    Keflex again. Talk about nauseous so off to have dry biscuits and yet more water. Told it’s more common as one gets older. That’s all I need now. Will be back if the Keflex works.

  2. Dorothy

    I started Cephlexin yesterday for a dental infection, 500 ml 3x a day. Diarrhea, nausea, migraine headache and joint pain within 10 hours. I am allergic to Penicillin and after reading just some of the warning out on the internet about this drug…disturbed it was prescribed to me in the first place. What does it actually help and do that even justifies its use!!!

  3. Mandy

    I was prescribed cephalexin for a uti. Within 24 hours I had severe stomach pain and had to stop taking it. After four days of pain a blood test indicated pancreatitis and I was admitted to hospital. A CT scan ruled this out and I have been diagnosed with a stomach ulcer. Never had one before, now been nearly three weeks and I’m still in pain, still taking narcotic pain relief, still having treatment and, incidentally, still itching all over my stomach.
    To add insult to injury the uti was discovered during a routine checkup. Had no idea…


    I finished this RX after taking it for 10 days for a UTI. I’m still having extreme tiredness and bloating with stomach pain. I don’t know why it’s taking so long to leave my body but I don’t ever want to take it again. I looked at the other meds for a UTI, and I can’t take any of them due to earlier side effects. I just hope I don’t get another UTI!!!

  5. Gordon

    I refuse to take Cephalexin, as side effects horrendously unpleasant. Must be another drug out there for infected oedema in one leg only.

  6. Raine

    I just posted, today, November 30, 2017. I forgot to ask- has anyone been given Keflex for a uti/kidney infection. I’m interested to know side effects and what and where the pain was regarding the kidney nfectiom. How much were you on and for how long. Outside of testing urine culture did anyone have to go for any tests, MRI, CAT SCAN?

  7. Therese

    Into fifth day of this antibiotic for UTI and have pain in my sinuses and headache as well
    as sneezing and sneezing. Have just 2 more days to continue, should I do so. I was ok for the
    first three days. I am on Warfarin, Bisoprolol and Amlodipine as well and have AF


    • Raine

      Woke up 4AM on November 30- today, with burning during urination. I knew it was a UTI. A couple of days before that my kidney, right side, was bothering me. Then the UTI appeared. This is my first day, and I feel extreme nausea. My right side kidney, not so bad. Today is Thursday. I have 500mg twice a day. If by Saturday not feeling better will go to see my doctor. I’ve taken different antibiotics for sinus, bronchial – never had side effects of nausea just a muscle weakness- tired. Never took Keflex before.

  8. Mary Wier
    chattanooga tennessee

    Dear PP I am so grateful for the comments on cephalexin as I have a bottle of 500 mg pills prescribed for possible infection of of ankle sore from marine larvae in hotel pool I read the warnings on the papers and got fearful to take it God bless you all for speaking out I am using emu oil from Songline in Maine on my sore it is healing from the inside Thanks from Mary inTennessee

  9. Eulala
    West Pelzer, South Carolina

    Does anyone who’s taken Cephalexin know how long the side effects generally last? I started taking Cephalexin a few days ago after the urgent care doctor prescribed them to me for an infection. I’ve been having a lot of nausea from the drug, but I know it’s not good to stop taking an antibiotic before finishing the full prescribed amount. Will the side effects last for the entire period of my taking them? Or will they eventually die off? I just want to know whether or not I need to go get some more phenagren in order to make it through this!

  10. EJ

    Having used Ibelix (Cephalexin) in the past, I was none too concerned when it was prescribed for multiple dental abscesses and a sinus infection. (Dosage: One 500mg capsule, three times daily.) While the initial side effects were tolerable (nausea, headache, jitteriness, sweats, changes to vision), the second and third courses were horrendous. Symptoms increased to include migraine, joint pain, mouth ulcers, peeling and swollen lips, facial swelling and numbness (which was advantageous for stopping the severe dental pain), difficulty swallowing, throat swelling, asthma and wheezing.

    Funnily enough, my general practitioner advised me to continue taking the course (s). However when I stated passing massive blood clots, I declined further treatment.

    While I knew for a fact that the afore-mentioned symptoms were related to Cephalexin exposure, convincing my medical practitioners (including a so-called allergy consultant) was another matter, as they refused to accept the association (even when the literature from the drug company themselves mentions the association!) I was however personally able to verify the link because I had kept detailed notes (and photographs) during the treatment period. I had also taken the liberty of trialing the removal and reinstatement of the drug. Eg. Each time the drug was removed, my symptoms disappeared within several hours. Likewise, each time the respective drug was reintroduced, the symptoms returned within a short timeframe.

    I have since discovered that (despite being a rarity) internal bleeding, numbness and swelling are indeed associated with Cephalexin usage. After perusing other comments within this section, I found I was not the only one to have discovered this link (TKS on February 3rd, 2014, Gina on December 12th, 2012, Tami on October 4th, 2011.)

    I recently discovered this drug is contraindicated for use with patients with heart conditions and Mastocytosis due to the risk of cardiovascular collapse. It has been my experience (and that of many others) that many doctors ignore obvious ‘allergic’ reactions in favour of continued treatment, despite the horrendous effect on patients. (Informed consent is vital!)

    While probiotics (containing ‘live’ bacteria) may alleviate gut symptoms, we need to learn to listen to our bodies and keep in mind that ALL allopathic drugs have side effects. At the end of the day, the drug in question did nothing to resolve any of the bacterial infections my body was plagued with and went on to cause oral and vaginal thrush. Incidentally, the facial numbness lasted for in-excess of four months, after discontinuing treatment.

    Given the serious nature surrounding potential side effects, I personally don’t believe this drug ought to be dispensed for minor ailments, particularly those that can be dealt with topically, as there are many more (less harmful) treatment options available. For those suffering from recurrent urinary tract infections (UTI), there are better solutions. Eg. D-Mannose, vaginal wiping (after toileting) with a mixture of Apple Cider Vinegar (Bragg’s Organic Raw Unfiltered), Bi-carb soda and water) and other dietary/lifestyle changes.

    Remember, at the end of the day, NO-ONE has a vested interest in our health (and survival) other than ourselves. Thus, our relationship with our medical practitioner (s) needs to be a ‘joint’ partnership and not one of subservience.

  11. Joanne D

    Giant hives like spider bites 45 minutes after taking Cephalexin for bacterial infection from athletes foot. Palms itching. Sprayed the lesions with Benadryl. I am done. If podiatrist can’t cure it with amoxicillin, the one antibiotic I know I can tolerate, then I am at a loss what to do. Not taking more of this, that is for sure. For all I know I am allergic to the hideous orange dye in these capsules. Mad chemists at work?

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