The American Cancer Society estimates that in 2021, over a quarter million new cases of invasive breast cancer will be diagnosed in women.

While chances for survival are increasingly good, breast cancer is still a serious disease that results in tens of thousands of deaths each year.

That’s why it’s more important than ever to understand the disease and how to treat it.

Scientists don’t know what causes breast cancer. They have identified several risk factors for the condition, but that’s not the same as causes.

For example, exposure to too much estrogen can increase the risk of breast cancer. This is because some breast cancer growth may be caused by high levels of estrogen, mostly made by the ovaries. (Estrogen isn’t the only risk factor for breast cancer, though.)

One treatment for people with higher amounts of estrogen is antiestrogen therapy.

This treatment reduces or blocks the amount of estrogen in the body in order to lower the risk of advanced-stage or metastatic breast cancer. Metastatic breast cancer is cancer that has spread to other parts of the body.

Here’s how antiestrogen therapy works and what you need to know about the risks and side effects.

Antiestrogen is a form of hormonal therapy that aims to slow or stop the growth of hormone-sensitive tumors.

The therapy works in several ways. It can block the body’s ability to produce cancer-causing hormones or it can change the way hormones affect breast cancer cells.

It can also be used on more advanced breast cancers to make them smaller in size or slow their growth.

Antiestrogen therapy is generally used alongside surgery that removes the tumor. Antiestrogen therapy will ideally prevent the cancerous tumor from coming back.

It’s typically taken as a daily oral pill, though sometimes it is taken as an injection instead.

Estrogen blockers are chemicals that essentially prevent cancer cells in females from getting the hormones they need to grow, like estrogen.

Many breast cancers are sensitive to hormones like estrogen and respond to this type of treatment. This accounts for 70 to 80 percent of all breast cancers, or 2 out of 3.

When breast cancers sensitive to hormones can’t get estrogen they need to grow, they may shrink in size or be unable to grow much at all.

However, a smaller number of breast cancers are insensitive to hormones, meaning they don’t benefit from antiestrogen therapy and require different treatments.

Estrogen suppressing therapy is a form of antiestrogen therapy that slows or reduces the amount of estrogen in a female’s body.

A group of estrogen suppressants called aromatase inhibitors, for example, block the enzyme aromatase, which turns the hormone androgen into small amounts of estrogen in the body.

This results in less estrogen being available elsewhere to contribute to the growth of breast cancer cells that are sensitive to hormones.

While estrogen suppressing therapies can’t stop ovaries from making estrogen, they can reduce the body’s production of estrogen.

Generally taken as a pill, estrogen suppressants are best for treating early stage breast cancer that’s sensitive to hormones. Joint stiffness and joint pain are two common side effects of the medication.

However, like all medications, estrogen suppressants may also have serious side effects. Heart complications and increased bone loss are rare but possible.

Both antiestrogen and estrogen blockers aim to slow or stop the progression of breast cancer. The two different treatments don’t work the same way, though.

Tamoxifen, a widely used estrogen blocker, stops estrogen from affecting breast tissue. Aromatase, an antiestrogen therapy, lowers the production of estrogen in postmenopausal women.

Tamoxifen is well known and widely used as chemoprevention — a medication to prevent disease — for women with a high risk of developing invasive breast cancer, according to 2019 research.

In a 2006 study, researchers found that when postmenopausal women in the increased risk category took tamoxifen for 5 years, it lowered their risk of developing invasive breast cancer by 50 percent.

A 2017 study found that use of tamoxifen and antiestrogen medications led to a 50 to 65 percent decrease in breast cancer development in high risk groups.

Both estrogen blockers and antiestrogen therapies are effective, but there are side effects, too. It’s important to weigh the benefits and risks with your care team before deciding on this course of action.

Side effects of estrogen blockers

Side effects of estrogen blockers can vary depending on the drug. The most commonly reported symptoms are:

  • hot flashes
  • night sweats
  • vaginal dryness
  • menstrual cycle changes (in premenopausal people)

More serious but less common side effects of estrogen blockers can include blood clots or increased risk of developing uterine cancer. Postmenopausal women face an even higher chance of a uterine cancer diagnosis.

A very rare but potential side effect of antiestrogen therapy is a stroke, so it’s essential not to ignore any sudden feelings of confusion, headaches, or trouble moving or speaking.

You’ll want to call 911 immediately if any of these issues arise while taking estrogen blockers.

Side effects of antiestrogen therapy

Side effects of antiestrogen oral therapies can also vary depending on the drug. The most commonly reported symptoms are:

  • joint pain
  • accelerated bone loss
  • heart problems

Short-term side effects of antiestrogen injection therapy, like the drug fulvestrant, may include:

  • night sweats
  • mild nausea
  • bone pain
  • hot flashes
  • vaginal dryness
  • pain at the injection site

Antiestrogen therapy can be an effective treatment for hormone-sensitive breast cancer in women. It can also potentially be used as a preventive measure for individuals at risk of developing the disease.

Serious side effects are rare, but it’s still important to consider the risks.