If you’ve had abnormal menstrual bleeding while taking tamoxifen (Soltamox), you’re not alone. People who take tamoxifen before menopause may experience menstrual changes. These changes can vary from person to person and may include heavier bleeding, lighter bleeding, irregular cycles, or periods that stop for some time.

Tamoxifen is a hormonal therapy that may be recommended for hormone receptor-positive types of breast cancer — in other words, when hormones like estrogen fuel the growth of breast cancer cells. Breast cancer itself does not usually cause heavy periods. More often, heavy bleeding is linked to breast cancer treatment, including hormonal therapy with tamoxifen, or to a separate gynecologic condition.

Tamoxifen may cause a range of side effects, including hot flashes, fatigue, vaginal discharge, 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 affect your daily life.

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

For some people, heavy menstrual bleeding during breast cancer treatment can happen while taking tamoxifen. Formerly called menorrhagia, these periods may last longer than usual or lead to additional symptoms, 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.”

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

• Fatigue (severe tiredness)
• Weakness
• Dizziness or lightheadedness
• Shortness of breath
• Chest pain
• Cold hands and feet
• Headaches
• Pale or yellowish skin, which may be easier to notice on lighter skin tones

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.”

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

Additionally, people may experience the opposite effect — their periods become lighter or less frequent. Tamoxifen may also make your period go away completely (called amenorrhea), although it’s likely to come back once you’re done taking this medication, unless you’ve gone through menopause. If you’ve gone through menopause and experience new vaginal bleeding on tamoxifen, call your healthcare provider.

“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.

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.

Bleeding changes, including spotting or blood in vaginal discharge, can indicate changes in your uterus. These changes may 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.”

Abnormal bleeding may also be a symptom of cancer of the uterus or cervix. Tamoxifen can raise your risk of developing endometrial cancer, but the risk is still very low. Because unusual vaginal bleeding can be an endometrial cancer symptom, it’s important to tell your healthcare team about any bleeding changes.

Because tamoxifen can sometimes affect the uterus, your healthcare provider may recommend checking your uterus if you have symptoms, such as new or unusual vaginal bleeding. Most of the time, testing is done to figure out the cause of symptoms, not as a routine test for everyone. If you notice bleeding that is new for you, tell your cancer care team and your gynecologist.

Checking the uterus may involve a transvaginal ultrasound, which can help your healthcare 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 healthcare provider may also screen for problems with an endometrial biopsy (removal of a small piece of tissue from the uterine lining 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 healthcare providers recommend screening tests during tamoxifen therapy in the absence of symptoms. Ask your healthcare provider whether they think screening for uterine problems may be a good idea while you’re on tamoxifen.

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

Testing may involve an in-office procedure in which your healthcare provider uses a small suctioning tube to perform an endometrial biopsy. In other cases, your healthcare provider may perform a surgical procedure called dilation and curettage (D&C) in an operating room. During a D&C, your healthcare provider 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.

You may not need to treat heavy or abnormal periods associated with tamoxifen use. However, you might want to, especially if the bleeding affects your life or comes with symptoms such as pelvic pain.

Certain types of birth control, such as progestin-containing intrauterine devices (IUDs), may help treat heavy bleeding. IUDs are inserted directly into the uterus.

Talk with your oncologist and gynecologist about what’s safest for you, and ask about nonhormonal options if you’re not sure. They may also reassess your tamoxifen treatment.

If you’re done having children or don’t want to become pregnant, you may also be eligible for a hysterectomy (surgical removal of the uterus). This procedure will eliminate your menstrual cycle and abnormal bleeding, but you won’t be able to become pregnant afterward.

Abnormal bleeding can be painful and frustrating. Talk to your healthcare provider if you experience this symptom with tamoxifen. Your provider can help make sure you’re not dealing with a serious health issue and can discuss the benefits and risks of different treatment options.

Join the Conversation

On MyBCTeam, people share their experiences with breast cancer, get advice, and find support from others who understand.

Do you take tamoxifen? Have you noticed a change in your periods since you started treatment? Let others know in the comments below.