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Selective Estrogen Receptor Modulators: What Are SERM Drugs?

Selective estrogen receptor modulators (SERMs) control the level of estrogen and its access to cells. They are prescribed for osteoporosis, menopause symptoms, and breast cancer therapy. Let’s take a look at how these hormone modulators work and what effect they have on your body. 

Selective estrogen receptor modulators (SERMs) are used to treat estrogen-related diseases like osteoporosis and breast cancer. They can block or activate estrogen. The word “selective” in their name means they can target different estrogen receptors in the body, such as in the breast, uterus, and bones. Once they reach their target, they selectively block or activate estrogen.

Not much is understood about the selective nature of selective estrogen receptor modulators or how they decide whether to block or activate estrogen.

For instance, in the case of breast cancer, selective estrogen receptor modulators will block estrogen by attaching to the estrogen receptors in breast tissue. This takes up the space reserved for estrogen and blocks the message to your brain to send more estrogen to the breast. This, in turn, stops the cancerous growth from increasing and reproducing.

Selective estrogen receptor modulators are also used to treat osteoporosis. When used in this case, SERM drugs target the bones and activate estrogen receptors. For people with osteoporosis, a selective estrogen receptor modulator can help restore bone mineral density and reduce their risk of fractures.  

Even though the structure of estrogen is different in different parts of your body, selective estrogen receptor modulators are able to identify and adapt to each situation. Estrogen receptors in your uterus are different from the ones in your bones, but SERMs are still able to find and interact with them. In the uterus, a selective estrogen receptor modulator can bind with the estrogen cells and activate more production. This can help alleviate symptoms of menopause.

Selective estrogen receptor modulators are highly adaptable and work for many different estrogen-related diseases. Raloxifene and tamoxifen are two commonly prescribed SERM drugs.

In the 1960s, before the invention of oral contraception, scientists believed that selective estrogen receptor modulators could be used for birth control. Although human trials showed that SERMs aren’t effective as oral contraception, these studies did uncover some surprising health benefits, including the reduction of bone density loss and the blocking of breast cancer cells.

Selective estrogen receptor modulators can provide benefits for both premenopausal and postmenopausal women. However, they are not without their risks. Because of this, doctors will often recommend starting with aromatase inhibitors, which don’t completely stop the ovaries from making estrogen, instead. 

Researchers have conducted several studies comparing aromatase inhibitors to selective estrogen receptor modulators, particularly tamoxifen. Aromatase inhibitors are typically prescribed in the following scenarios: 

  • Early stages of breast cancer in receptive cases
  • Continuing hormone replacement after taking tamoxifen for two to three years
  • As part of a cancer treatment plan (five years of tamoxifen followed by five years of aromatase inhibitors)

Doctors currently think that the best hormone replacement therapy for treating estrogen-related cancers is a combination of SERMs and aromatase inhibitors. They will often prescribe tamoxifen for a maximum of five years, followed by an aromatase inhibitor.

For postmenopausal women younger than 45, SERMs are often prescribed to combat the symptoms of menopause caused by changes to blood vessels, including hot flashes, night sweats, bone loss, and vaginal changes. Suddenly stopping your SERM drugs can mean a return of these symptoms, with increased intensity and severity. 

Weaning yourself off selective estrogen receptor modulators needs to be done gradually and under the careful supervision of your doctor. Whether you’re switching to aromatase inhibitors or you’ve reached the end of the five-year course, you’ll need a post-cycle therapy plan so your symptoms don’t suddenly return. Ask your doctor for your best options for post-cycle therapy after SERMs. It can take as long as three months to cycle off your SERM.

Selective estrogen receptor modulators are often prescribed instead of traditional hormone replacement therapy because of the risks involved. Hormone therapy can increase your chances of breast cancer, endometrial cancer, thromboembolism, and vaginal bleeding. For women with a higher risk of developing these cancers and conditions, a selective estrogen receptor modulator is a possible option. 

There are two primary beneficial functions of SERM drugs for postmenopausal women: 

  1. They activate estrogen receptors in the bones to prevent bone loss and help build mineral density.
  2. They block estrogen receptors in breast tissue to prevent cancer cells from developing. 

Selective estrogen receptor modulators also target estrogen receptors in the uterus and help with vaginal dryness and atrophy.

In the treatment of osteoporosis, selective estrogen receptor modulators like tamoxifen and raloxifene increase estrogen production in the bone tissue. This can help maintain the structure and strength of your bones and reduce mineral depletion. With improved bone mineral density, you’re less likely to experience a fracture.

To determine if you’re a candidate for SERM drugs for osteoporosis, your doctor will do a bone scan to test your bone mineral density and review your medical history for any other risk factors. 

Many postmenopausal women consider taking medication to prevent osteoporosis, and SERM drugs are a possible option. Not only can SERMs help reduce your fracture risk and strengthen your bones, but they can also decrease your chances of developing breast cancer.

If you’re at a high risk of developing breast cancer, you may have been prescribed endocrine therapy, which can control the spread of cancer and keep it from coming back. Scientists have also found that SERMs can be effective at preventing breast cancer for all women. By binding to the receptor sites and blocking estrogen-positive cancer cells, SERMs can stop breast cancer’s growth and development.

For women with endometriosis, selective estrogen receptor modulators can sometimes help. However, clinical trials have shown that using tamoxifen for a long time can lead to a higher risk of developing endometrial hyperplasia and uterine cancer. In the uterus, tamoxifen activates estrogen production, increasing the risk of endometrial health issues. In contrast, raloxifene targets the uterine cells and blocks the production of estrogen. Because of this, raloxifene may be a better option for treating endometriosis.

While many women find SERMs to be beneficial to their health, the drugs do have risks. Make sure to talk to your doctor about the risks and potential benefits of selective estrogen receptor modulators before deciding if they are right for you. For example, if you are a smoker or have a history of blood clots or heart attack, SERM drugs might not be a good choice.

Selective estrogen receptor modulators are not suitable for breastfeeding women or if you’re trying to get pregnant. SERMs can have detrimental effects on the development of a fetus. While taking SERM drugs, consider using non-hormonal contraception like condoms or a copper-based IUD to avoid throwing off the balance of your hormones.

Some more serious side effects of selective estrogen receptor modulators include blood clots, stroke, and endometrial cancer. 

If you are taking SERMs, you can expect to experience some of these common side effects:

  • Fatigue
  • Hot flashes
  • Night sweats
  • Vaginal discharge
  • Mood swings

More extreme side effects to watch out for include:

  • Unusual vaginal bleeding or discharge
  • Pain or pressure in your pelvis
  • Leg swelling or tenderness
  • Chest pain
  • Shortness of breath
  • Weakness, tingling, or numbness in your face, arm, or leg
  • Sudden vision trouble
  • Dizziness
  • Sudden and severe headache

If you experience any of the side effects listed above, see your doctor right away.

Selective estrogen modulators have been shown to have real health benefits for premenopausal and menopausal women. They are often prescribed to treat osteoporosis, breast and endometrial cancers, and some symptoms of menopause. Ask your doctor about SERMs and discuss the risks and benefits before deciding if this hormone therapy is right for you.

Harold N Rosen, MD. Selective estrogen receptor modulators for prevention and treatment of osteoporosis: UptoDate; Nov 11, 2019.

Breastcancer.org: Selective Estrogen Receptor Modulators (SERMs); August 15, 2018

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Hum Reprod Update, May-Jun 2000; 6 (3):244-54. DOI: 10.1093/humupd/6.3.244

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