If you’ve had abnormal bleeding while taking tamoxifen (Soltamox), you’re not alone. More than half of premenopausal women who take this drug experience menstrual changes, including heavy periods, when it becomes part of their breast cancer treatment plan.

Tamoxifen is a hormonal therapy that may be recommended for hormone receptor-positive types of breast cancer — in other words, hormones like estrogen fuel the growth of breast cancer cells.

Unfortunately, tamoxifen is notorious for causing side effects including a metallic taste, hot flashes, tiredness, and nausea, as well as changes to your period. However, the experience of using this drug varies greatly from person to person. You may notice only a single mild side effect, or you may develop several that disrupt your daily life.

Here are some important things to know about how tamoxifen may affect your period.

1. Tamoxifen May Lead to Heavy Periods

Heavy menstrual periods, also called menorrhagia, may cause you to bleed much more than you did during previous menstrual cycles. These periods may also last longer than usual or lead to additional side effects, such as cramping and passing blood clots. Heavy periods are a possible side effect of tamoxifen.

Some members of MyBCTeam have talked about their experiences with heavy bleeding while using tamoxifen. One member wrote, “I feel my cramps are terribly painful, and my period is just heavier overall. I woke up this morning and my PJs had a significant amount of blood on them.”

2. Heavy Periods Can Cause Additional Health Problems

If you lose too much blood through your periods, you could develop anemia (low levels of red blood cells). Anemia, in turn, can cause symptoms such as:

• Fatigue
• Weakness
• Dizziness
• Shortness of breath
• Nausea
• Pale skin
• Chest pain

Anemia may also lead to a fast or irregular heartbeat, which can damage your heart over time.

A MyBCTeam member who used tamoxifen in the past posted, “[My periods were] so bad and heavy, I was anemic and getting nausea from just trying to function.”

3. Not Everyone Experiences This Side Effect

Some people don’t notice any difference in their periods once they start using tamoxifen.

Additionally, many people experience the opposite effect — their periods become lighter or less frequent. Tamoxifen may also make your period go away completely, although it’s likely to come back once you’re done taking this medication unless you’ve gone through menopause. These changes are more common than heavy bleeding, affecting about half of premenopausal women who take tamoxifen, according to a study in Mayo Clinic Proceedings. Abnormal bleeding patterns generally disappear once you stop using tamoxifen unless you go through menopause while taking this drug.

“I have been on tamoxifen for four years and have never had a period,” reported one member.

“My periods stopped as soon as I started taking tamoxifen. I was told to expect this,” another member added.

If you’ve already gone through menopause and no longer get periods, you won’t have to worry about this side effect. However, while tamoxifen can be used both before and after menopause, other hormone therapies, such as aromatase inhibitors, are more commonly recommended for those who’ve already gone through menopause.

4. You Should Tell Your Doctor About Increased Bleeding

Bleeding changes, including spotting or blood in vaginal discharge, often indicate changes in your uterus. These changes include polyps or fibroids (abnormal growths that usually aren’t cancerous) and hyperplasia (a thickening of the uterine lining that can sometimes lead to cancer).

“[My period] stopped for eight months and then returned with a vengeance,” a MyBCTeam member said. “That lasted four months. Then, my period was gone again after a polyp removal.”

In some cases, abnormal bleeding may be a symptom of endometrial cancer (cancer of the uterus). Having breast cancer can raise your risk of developing endometrial cancer. Additionally, some research shows that your risk of being diagnosed with this cancer is slightly increased when you take tamoxifen.

The vast majority of people who use this medication don’t develop additional cancers, and abnormal bleeding is usually a treatment side effect rather than a sign of endometrial cancer. However, it’s still a good idea to tell your health care team about changes in vaginal bleeding so they can rule out more serious issues.

5. Some Providers Recommend Screening for Uterine Conditions

Because tamoxifen can sometimes lead to uterine changes, your oncology team may suggest regular screenings while you take this drug. These tests can help your doctor look for changes in your uterus, even if you’re not having symptoms such as heavy bleeding. In one study, about half of people with breast cancer who used tamoxifen treatment said they underwent screening.

Screening often consists of a transvaginal ultrasound, which can help your health care team view the organs in your pelvis. During this test, a technician places a transducer (a long, thin device) into your vagina and uses harmless, painless sound waves to look at nearby tissues. Your doctor may also screen for problems using an endometrial biopsy (removal of a small piece of uterine tissue to be examined under a microscope).

One MyBCTeam member wrote that they had biopsies done every six months while on tamoxifen. “All has been benign and it’s been going on for a couple of years now,” they said. Another member replied, “I get monitored every couple of months with intravaginal ultrasounds. It is easy for them to tell if there are any abnormalities.”

Not all providers recommend screening tests during tamoxifen therapy in the absence of symptoms. Ask your doctor whether they think screening for uterine problems may be a good idea while you are on tamoxifen.

6. Simple Procedures Can Rule Out Uterine Problems That May Cause Bleeding

If you experience increased bleeding while using tamoxifen, your doctor will likely want to perform additional tests to more closely examine the tissue of your uterus. You should always tell your doctor about bleeding while on tamoxifen.

In many cases, testing involves a simple in-office procedure in which your doctor uses a small suctioning tube to perform a biopsy. In other cases, your doctor may perform a surgical procedure called dilation and curettage (D&C). During a D&C, your doctor dilates (widens) your cervix and removes a larger tissue sample that can be checked for signs of cancer.

“When I had heavy bleeding,” one member commented, “my gyno did check this because it wasn’t normal for me. Everything turned out OK, but I think it was part of my hormones going wacko from the meds.”

“My oncologist told me if I do bleed heavily to come back in,” another member shared.

7. Abnormal Bleeding May Be Treatable

You don’t have to treat heavy or abnormal periods caused by tamoxifen use. However, you might want to, especially if your bleeding is inconvenient or embarrassing or disrupts your life.

Certain types of birth control may help treat heavy bleeding. In particular, intrauterine devices (IUDs) may be a good idea. These T-shaped pieces of plastic are inserted directly into the uterus. Mirena — a type of IUD that releases a hormone called levonorgestrel — can lessen heavy menstrual bleeding, although it typically takes several months to start working. Twenty percent of the time, menstrual periods stop altogether after Mirena has been in place for at least a year, according to Mayo Clinic. This IUD is safe to use while taking tamoxifen.

If you are done having children or don’t want to become pregnant, you may also be able to have a hysterectomy (surgical removal of the uterus). This will eliminate your menstrual periods and abnormal bleeding, but you won’t be able to become pregnant after this procedure.

Abnormal bleeding can be a pain, both literally and figuratively. Talk to your doctor if you experience this symptom. Your provider can help ensure you’re not dealing with a serious health issue and can discuss the benefits and risks of different treatment options.

Finding Support

MyBCTeam is the social network for people with breast cancer and their loved ones. On MyBCTeam, more than 64,000 members come together to ask questions, give advice, and share their stories with others who understand life with breast cancer.

Do you take tamoxifen? Have you noticed a change in your periods since you started treatment? Share your experiences in the comments below, or start a conversation by posting on your Activities page.