Questions and Answers About Tamoxifen

1. What is tamoxifen?

Tamoxifen is a medication in pill form. It interferes with the activity of the female hormone estrogen. For more than 20 years, it has been used to treat advanced breast cancer. It has recently been used as adjuvant therapy for early stage breast cancer. That’s treatment that’s used along with other treatment. In this case, it’s used after surgery or radiation. Studies also show it can help prevent breast cancer in women at high risk for this disease. And studies are still going on to see how it works in the prevention of breast cancer. There are also studies looking at how it might be used to treat other types of cancer.

2. How does tamoxifen work on breast cancer?

Estrogen promotes the growth of breast cancer cells. Tamoxifen is often called an "anti-estrogen." It works against the effects the hormone has on these cells. The drug slows or stops the growth of cancer cells that are already in the body. As added therapy, it has been shown to help prevent the original breast cancer from returning. It also prevents the development of new cancers in the other breast.

3. Are there other benefits from tamoxifen?

Tamoxifen counters the effects of estrogen in breast tissue. At the same time, it acts like the hormone in other parts of the body. This means that if you take it you may get many of the benefits that come with estrogen replacement therapy. Those include lowering your blood cholesterol. They also include slowing bone loss, or osteoporosis.

4. Can tamoxifen prevent breast cancer?

Research shows that when tamoxifen is used with other treatments for early stage breast cancer there are two major benefits. First, it prevents the recurrence of the original cancer. It also prevents new cancers in the other breast. The NCI funded a large research study. It was called the Breast Cancer Prevention Trial (BCPT). The goal was to determine how useful tamoxifen is in preventing breast cancer in women who are at higher risk for the disease. Results showed a 49 percent reduction in invasive breast cancer among women who took it. Women who took it also had 50 percent fewer noninvasive breast tumors. There are, though, some risks with tamoxifen. Some of them are even life threatening. The choice to take tamoxifen is an individual one. You and your doctor need to carefully consider both the benefits and the risks of therapy.

At this time, there is no evidence that says tamoxifen is beneficial for women who do not have an increased risk of breast cancer.

5. What is the Study of Tamoxifen and Raloxifene (STAR)? How can a woman learn more about it?

STAR was a clinical trial that involved nearly 20 thousand women. The women had all already gone through menopause. The trial compared outcomes related to the use of tamoxifen and raloxifene. The main outcome measures included the incidence of:

  • invasive breast cancer
  • uterine cancer
  • noninvasive breast cancer
  • bone fractures
  • thromboembolic events (blocking of a blood vessel by a particle from a blood clot)

The results for breast and uterine cancer are shown in the following table.

Outcome Tamoxifen Raloxifene
Invasive breast cancer 163 cases 168 cases
Noninvasive breast cancer 57 cases 80 cases
Uterine cancer 36 cases 23 cases

No differences were found for:

  • other invasive cancers
  • ischemic heart disease events
  • stroke
  • fractures related to osteoporosis
  • total number of deaths
  • cause of death

There were less thromboembolic events in the raloxifene group. There were also fewer cataracts and cataract surgeries in that group.

The trial showed that the two drugs were equally effective in reducing the risk of invasive breast cancer. It also showed that raloxifene has a lower risk of events caused by blood clots and cataracts. The difference in the rate of noninvasive breast cancer was not statistically significant. You should talk with your doctor about which drug may be better suited for you.

6. Does tamoxifen cause blood clots?

Studies show there is a small increase in the number of blood clots in women taking tamoxifen. This risk is greater if a woman is taking chemotherapy along with tamoxifen. The total number of women who have had this side effect is small. The risk is similar to the risk of blood clots from estrogen replacement therapy.

7. Does tamoxifen cause uterine cancer?

In the BCPT, women taking tamoxifen had more than twice the risk of uterine cancer than women taking placebo. The risk was the same as (or less than) the risk from estrogen replacement therapy. More studies are under way to examine the role other risk factors for uterine cancer play in women taking tamoxifen.

Uterine cancer can be life threatening. Most cases that have occurred during studies of tamoxifen have been found early. Treatment was usually effective. Breast cancer patients, though, who developed uterine cancer while taking tamoxifen have died from the disease. Two key symptoms are abnormal vaginal bleeding and pelvic pain. If you take tamoxifen, see your doctor if get these symptoms.

8. Does tamoxifen cause eye problems?

As you age, you are more likely to develop cataracts. A cataract is a clouding of the lens inside your eye. Women taking tamoxifen appear to be at increased risk for cataracts. Other eye problems have also been reported in a few patients. These include corneal scarring or retinal changes.

9. Does tamoxifen cause other types of cancer?

There have been a few reports of liver cancer. There have also been reports of other liver toxicities. Tamoxifen can cause liver cancer in certain strains of rats. But it is not known to cause liver cancer in humans. It did not cause liver cancer in the BCPT. It can sometimes cause other liver toxicities in women. But those are rarely severe or life threatening. You doctor may order blood tests from time to time to check liver function.

One study suggested a possible increase in cancers of the digestive tract related to tamoxifen. But other trials, including the BCPT, have not shown a relationship between tamoxifen and these cancers.

Studies such as the BCPT show no increase in cancers other than uterine cancer. This risk is being evaluated.

10. Should a woman taking tamoxifen avoid getting pregnant?

Yes. Tamoxifen may make some women more fertile. But doctors advise women who take it to avoid getting pregnant. Animal studies suggest that the drug can harm the fetus. If you have questions about fertility, birth control, or pregnancy, discuss them with your doctor.

11. What are some of the more common side effects of taking tamoxifen?

The side effects are similar to some of the symptoms of menopause. The most common ones include hot flashes and vaginal discharge. Other side effects include:

  • irregular menstrual periods
  • dizziness
  • headaches
  • fatigue
  • loss of appetite
  • nausea
  • vomiting
  • vaginal dryness
  • vaginal bleeding
  • irritation of the skin around the vagina

Not all women who take tamoxifen have these symptoms.

12. Does tamoxifen cause a woman to begin menopause?

Tamoxifen does not cause a woman to begin menopause. It can, though, cause some symptoms that are like those that occur during menopause. In most women taking tamoxifen before menopause, the ovaries continue to act normally. They produce female hormones—estrogens—in the same or slightly increased amounts.

13. Do the benefits of tamoxifen in treating breast cancer outweigh its risks?

The benefits of tamoxifen as a treatment for breast cancer are firmly established. They far outweigh the potential risks. It’s right to be concerned about the risks and benefits of any medications you take. You should discuss those concerns with your doctor.

14. How long should a woman take tamoxifen for the treatment of breast cancer?

Women with advanced breast cancer may take tamoxifen for varying lengths of time. It depends on, among other things, how they responded to prior treatment. When it’s used as adjuvant therapy for early stage breast cancer, it is generally prescribed for 5 years. The ideal length of treatment with tamoxifen, though, is not known.

Two studies have confirmed the benefit of taking tamoxifen daily for 5 years. These studies compared 5 years of treatment with 10 years of treatment. When taken for 5 years, the drug prevents the recurrence of the original breast cancer. It also prevents the development of a second primary cancer in the other breast. Taking tamoxifen for longer period years is not more effective than 5 years of therapy.