The first big advance in the fight against recurrent breast cancer was the drug tamoxifen (Nolvadex). In September 1998 a major study demonstrated that breast cancer recurrence could be reduced with tamoxifen. Within one month the FDA had approved the drug. During the intervening years the data have been remarkably consistent: taking tamoxifen for 5 years post diagnosis reduces a woman’s risk of estrogen receptor (ER)-positive breast cancer recurrence by about 50%.

The big unanswered question, however, has been how long should a woman continue to take this drug. Until this week the answer has been 5 years. But new and compelling data suggest that 10 years might be a better target. The study was published in The Lancet (Dec. 5, 2012). Researchers followed nearly 13,000 women who had ER-positive breast cancer for 15 years. One group of women took tamoxifen for five years and then were switched to placebo. Another group received tamoxifen for 10 years. Those who received the drug for 10 years had a 50% relative reduction in breast cancer recurrence compared to those who stopped after five years.

It is always best to translate relative risk into absolute terms so you can actually understand the big picture. Over the 15 years of the study, those taking tamoxifen for five years had a 15 percent chance of dying from breast cancer. Those who took tamoxifen for a decade had a 12.2 percent chance of dying from this disease. That’s significant (an absolute risk reduction of dying of 2.8%) but doesn’t sound as impressive as a 50 percent relative risk reduction.


This drug is an anti-estrogen in the breast. Think of it a bit like bubblegum. If you jam a wad of bubblegum into the lock of a door you won’t be able to get the key in to open the door. The door will remained locked and shut.

Tamoxifen fits into estrogen receptors in breast tissue and, like bubblegum, jams the metaphorical locks. The drug thereby prevents estrogen from occupying the receptor sites. Because estrogen cannot activate the receptors, the doors to breast cancer remain closed. Another benefit of tamoxifen is that it acts like estrogen for bone and reduces the risk of osteoporosis.

This sounds like a great story. The only trouble is that tamoxifen also acts like estrogen on the uterine lining. Instead of blocking estrogen there, it activates the tissue as if it were estrogen. Roughly 3 percent of the women who took tamoxifen for 10 years developed endometrial cancer. That was approximately double the rate of those who only took tamoxifen for five years. So, you sort of rob Peter to pay Paul. You can reduce your risk of breast cancer by 50 percent but you double your risk of endometrial cancer.

Some breast cancer experts have called this tamoxifen study a game changer and will recommend that all ER-positive breast cancer patients stay on tamoxifen for 10 years. Others are saying it represents a modest benefit and risks must be taken seriously.


Many women find this drug hard to handle. You will see in the list below some of the most serious and most uncomfortable complications to tamoxifen therapy:

  • Hot flashes, night sweats, menopausal symptoms
  • Blood clots, stroke
  • Reduced mental clarity, confusion, impaired memory
  • Cancer of the uterine lining (endometrial carcinoma), fibroids
  • Vaginal bleeding, vaginal dryness, painful intercourse
  • Digestive upset, nausea, vomiting, loss of appetite
  • Dizziness, lightheadedness, vertigo
  • Visual changes, blurriness, difficulty reading, cataracts
  • Fluid retention
  • Thinning of hair, hair loss
  • Sexual side effects, reduced libido
  • Blood disorders
  • Liver damage
  • Skin rash (requires immediate medical attention)

There is no easy answer when it comes to long-term treatment with tamoxifen. For women who can take the drug without experiencing side effects, it is a blessing. Reducing the risk of breast cancer recurrence and death is an important benefit. For those who experience lots of uncomfortable side effects, or something more serious such as a blood clot or endometrial cancer, the risks may be too great. Any decision to continue (or resume) treatment up to 10 years will require thoughtful conversation between a breast cancer expert and the patient.

We hope this information will facilitate this process.

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  1. Brenda
    Pasco wa

    I was 49 when I got breast cancer and I told the dr to just take them both, I don’t need or want them. It had spread to 3 of my lymph nodes so I had chemo!
    In reading your article my question is..if my breasts were removed (down to my rib cage) I don’t feel like I can get breast cancer again, does tomoxifen reduce cancer from coming Back in my lungs, liver, brain, ect?
    I stopped taking it after a year! I FINALY felt like myself again!

  2. C.Y

    Just finished 5 years of Tamoxifen. I gained 20lbs during those years despite exercise and a healthy diet. I suffered severe leg, hip, ankle cramps throughout. My cramping and hot flashes have stopped after less than a week. Hoping the swollen hands and weight gain will subside also. Will keep you posted.

  3. HelenM

    I am 77 now, in a couple of months it will be 8 years since my lumpectomy for DCIS and 33 radiation treatments. At the time, I was still experiencing all the effects of menopause, even tho I had a hysterectomy in 1987. I did use ERT, for years after that, until I began weaning down and then off by the time I was 65. Years and years of hot flashes, night sweats, weight gain, that continued after the lumpectomy. I tried tamoxifen, the flashes became permanent, I was flashing one after another, with barely a moment inbetween. With weather over 100 degrees common here in the summer, I was always miserable. Along with the hot flashes I felt as tho the weight of the world was on my back. Then there was the mustache, the chin hairs, it was all too much. At some point quality of life outweighs quantity of life. I stopped the tamoxifen. Began taking an herbal called change o life. Oh, so much better! I stopped it when I felt I no longer needed it; but now I am having hot flashes again, think I will once again use it.

    Genetic testing was not done in 2007; now it can be used so that those who will not be helped by these various medications do not have to take them and suffer with side effects, no benefit. In fact, today, many doctors view DCIS as cancer stage zero and adopt a wait and see program.

  4. Jane S

    Just completed 5yrs of tamoxifen. stage 2 breast cancer node negative her 2 positive. Took 17 rounds of herceptin, no chemo. I am 70yrs old. While on tamoxifen I have experienced terrible back and joint pain, a heavy discharge, also 2 D &C’s, not to mention the mood swings and developing high blood pressure,the list goes on… Dr now recommends 5more yrs therapy arimedex along with prolia or tamoxifen that increase chances of urine cancer. I have heard horrible stories taking arimedex….I’m leaning to opt out both options for better quality of life..

  5. meri

    I am 38 years old and I had DCIS on my left breast. So I decided to undergo total mastectomy and lumpechtomy. I started tamoxifen 2 months ago and yesterday I quit. I had troubles with my heart rhythm and I was restless all the time. I do not regret because I want to have a qualitative life.

    • Dee

      I was diagnosed November 14th, 2014 with grade 1, Stage 1, ER+, PR+ Breast cancer. I underwent a lumpectomy and had two nodes removed that were negative. I had 7 weeks of radiation and in March started Tomaxifen. I have been on this nasty drug for 3 months now and I am already experiencing 10 side effects. I am miserable! I meet with the onco tomorrow. I am truly thinking about stopping it, it is a very hard decision. I am 49.

      • Julie

        Did you stop the tamoxifen and how are you feeling now?

        • antionettec

          Help! Can’t find any information regarding this question. Can a women take Tamoxifen for a new primary breast cancer? I had DCIS in 1999, TX was lumpectomy, radiation and 5 years Tamoxifen. No side effects. Just diagnosed with a new cancer /Same breast, February 2016. IDC, STAGE 1A NO NODE INVOLVEMENT, Masectomy. er positive and her2neu negative. Haven’t started meds yet. Waiting for Oncotype dx results.

  6. TG

    I was on Tamoxifen for 5 years when I was 43 with very little side effects, or so I thought at the time. Its now that I look back that I do have a problem with mental clarity, confusion and memory loss. Does anyone know if this will improve now I have been off tamoxifen for 2 years. Although my consultant is talking about going back on it for another 5 years.

  7. Anne

    I’m 31, pre-menopausal and been on tamoxifen 4 years this July. I was on zoladex for 18 months in conjunction with tamoxifen at the beginning. I simply have had enough to be honest. I cannot find anything online about the difference in results between 4 years and 5 years and I really feel like I need to stop them at 4 years because my quality of life is being seriously affected.
    I’m having problems like ovarian cysts, major weight gain, damage to my vision, night sweats, insomnia, memory loss, major loss in libido, pain during intercourse… the list goes on. I’ve already discussed coming off tamoxifen a year early with my oncologist and she advised I stick it out for 1 more year but she won’t give me a hard time if I stop now. There’s not a chance in hell I could manage 10 years.

  8. Ann R.

    Well, I am just about to start taking Tamoxifen and have reservations about taking it at all. I am 83, in good health generally (I eat well and exercise), but I have a very tricky digestive situation. In fact I react to even 1/2 an Excedrin. Wheee high! Took antibiotics for an infection and they felled me like a tree. My question is: Hasn’t the estrogen in my body about disappeared by now??? So let’s go ahead with a lumpectomy and keep me under surveillance for a few months WITHOUT further medication or surgery. I want to have some quality of life in my final years, i.e., no nausea, night sweats, hair loss or whatever.

  9. Margie

    I took Tamoxifen for 5 years and then Femara for 5, after which I developed cancer again on the same breast. Any comment on the effectiveness of Femara and the requirement to only take that drug for 5 years also?

  10. Donna

    For me the recommended protocol was 5 yrs. of tamoxifen, followed by 5 of an aromatase inhibitor (primarily arimidex, after trying femara and aromasin). Do you know how that might compare to 10 yrs. of tamoxifen?

  11. Cathy

    I took Tamoxifen for five years and I would not start it again because these numbers are not convincing. Moreover, I read these types of reports with skepticism because of what I have read and heard on The People’s Pharmacy as well as in other sources: side effects are not systematically or well-tracked, and special interests often influence how results are reported in test reports, journal articles and the popular media.
    Even if the numbers were better, before I would restart Tamoxifen I would research in more detail why taking a strong drug, with side effects that are more than just discomforts, for so long is really justified in my particular case.

  12. Penny H.

    One of the worst side effects of Tamoxifen that nobody ever mentions is failure to heal. After my first breast cancer I took Tamoxifen. I decided to have a breast reduction on the non cancer side so I would be a little more even. The surgery didn’t heal and part of the graft literally fell off. One of the ladies in my lumpectomy support group who was also taking Tamoxifen had a hysterectomy. The incision didn’t heal and she had to stop taking Tamoxifen. My experience was bad enough but can you imagine having an incision for a hysterectomy that wouldn’t heal.
    I asked my oncologist if I could stop the Tamoxifen (I was 4 1/2 years out) and my breast started healing. Then the Dr. found a suspicious dimple on my cancer side and I had developed another cancer at exactly the same place as the first cancer so I had a double mastectomy. By this time Aromasin was on the market and I took it for five years with no problems except for night sweats and hot flashes. My oncologist gave me Megestrol and the menopause symptoms went away.
    Dear Penny, We are very grateful for your message above. There is very little information about delayed wound healing in standard drug information resources. The official prescribing information does not mention this problem.
    Patients reporting their experiences often reveal side effects that may not show up in clinical trials. We did find one reference to delayed wound healing but we suspect very few oncologists regularly review journals in plastic surgery (see below):
    The take home message is that patient power is for uncovering unexpected reactions to medications. Thank you for telling us about this problem.

  13. EC

    Thank you for clarifying the actual reduction or the risk of dying from breast cancer over a 5 or 10 year period. An actual 2.8% reduction, while accurately billed as a 50% reduction, sounds great. But in actuality, it is only a 2.8% reduction! And the risk of developing endometrial cancer doubles! To me this is not worth the reduction or the trade off. I stopped taking tamoxifen because of the side effects and never have regretted the decision.

  14. C.F.

    UGH…Not another 5 years…The side effects suck as it is…I am only 2 1/2 years in, and can’t even imagine 10 years…

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