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Rx pill bottle: Anastrozole side effects and how to avoid them
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Anastrozole side effects and how to avoid them

Anastrozole side effects include pain, weight gain, hair loss, and mood changes
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By | Oct. 3, 2022 Medically reviewed by Kristi C. Torres, Pharm.D.

Common anastrozole side effects | Serious side effects | Pain | Weight gain | Hair loss | Mood changes | Side effects timeline | Contraindications | Warnings | Interactions | How to avoid side effects | How to treat side effects

Anastrozole is a generic prescription drug used to treat breast cancer. It is also the active ingredient in the brand-name medication Arimidex. Both brand-name and generic anastrozole are used to treat postmenopausal women with hormone receptor-positive breast cancer. However, healthcare professionals sometimes prescribe it off-label to prevent breast cancer, treat early breast cancer in men or premenopausal women, or as part of fertility treatments. 

Anastrozole treatment typically lasts many years—as many as 10—to make sure cancer does not return. Called an aromatase inhibitor, anastrozole blocks an enzyme that converts hormones produced by the adrenal gland into the female hormone estrogen. Because estrogen attaches to hormone receptor-positive cancer cells and induces the tumor to grow, reducing estrogen levels helps fight back the cancer. Like all therapies that affect hormones, however, anastrozole has side effects that occur in most people taking the drug.

Common side effects of anastrozole

Hot flashes and flushing are the most common side effects of anastrozole. From most likely to less likely, anastrozole’s most common side effects are:

  • Hot flashes
  • Flushing
  • Nausea
  • Fatigue
  • Weakness
  • Joint pain
  • Infection
  • Depression
  • Headache
  • Back pain
  • Fluid retention and swelling
  • Elevated blood pressure
  • Trouble breathing
  • Constipation
  • Elevated cholesterol levels
  • Skin rash
  • Breast pain
  • Urinary tract infection
  • Cough
  • Trouble sleeping
  • Diarrhea
  • Stomach pain
  • Vomiting
  • Weight gain
  • Dizziness
  • Acid indigestion
  • Bone pain
  • Loss of appetite
  • Flu
  • Dry mouth
  • Anxiety
  • Muscle pain
  • Stuffy nose and sinuses
  • Sore throat
  • Weight loss
  • Tingling and numbness
  • Pelvic pain
  • Vaginal bleeding

Serious side effects of anastrozole

In order of incidence, the most serious side effects of anastrozole are:

  • Broken bones
  • New tumors or malignancies
  • High blood pressure
  • Chest pain
  • Osteoporosis
  • Cataracts
  • Liver problems 
  • Blood clots
  • Anemia
  • Heart attack
  • Endometrial cancer
  • Stroke
  • Severe allergic reactions
  • Stevens-Johnson Syndrome

Anastrozole and pain

Various aches and pains are common side effects of anastrozole treatment. Generalized pain, joint pain, back pain, abdominal pain, breast pain, muscle pain, and bone pain were all reported by participants in clinical trials at rates ranging from 8% to 17%. Now, nearly the same number of people not taking anastrozole also had these pain problems, but it is clear that anastrozole and an alternative aromatase inhibitor, Femara (letrozole), can cause pain as a side effect. Report any pain to the prescribing healthcare provider especially if the pain if the pain changes dramatically, is severe, unrelenting, or persistent.

Anastrozole and weight gain

Weight gain is a reported side effect of anastrozole, but it’s uncertain if the drug is to blame. In clinical trials, participants taking anastrozole for five years reported weight gain at the same rate as people not taking anastrozole. That suggests there may be other reasons for the weight gain. A majority of women do gain weight after a breast cancer diagnosis. If appetite increases or body weight goes up unintentionally, then get medical advice from the prescriber or a nutritionist.

Anastrozole and hair loss

Both hair loss and hair thinning are reported side effects of anastrozole, but the incidence is unknown. One study suggests that the risk of either hair loss or hair thinning is more than twice as likely in women with breast cancer if they’ve been taking an aromatase inhibitor like anastrozole or letrozole for two years or more. The incidence could be as high as 1 in 5 women. When on long-term anastrozole therapy, women should be aware that their hair could get thinner. There are effective treatments for this hair thinning, so get medical advice from a healthcare professional before deciding to stop the drug. 

Anastrozole and mood changes

Depression and anxiety are listed as side effects of anastrozole, but clinical trials showed that the incidence was about the same for women taking the drug as for women given an inert placebo. However, it is normal for people diagnosed with cancer and undergoing cancer treatment to feel depressed, anxious, or both. No matter what the cause, depression, anxiety, and other mood disturbances can be helped, so talk to a healthcare professional about treatment options. 

How soon do anastrozole side effects start?

Some of anastrozole’s side effects can begin in the first few days after starting treatment such as hot flashes, digestive system problems, headache, insomnia, and dry mouth could happen early in treatment. Some will get better after a few months. Most other side effects like weight gain, various body pains, hair loss, depression, and most severe side effects are delayed side effects that typically manifest weeks, months, or even years after the initiation of treatment.

How long do anastrozole side effects last?

Many of the milder side effects of anastrozole typically improve in the first weeks or months of treatment. Side effects that don’t improve during treatment typically go away once treatment ends. It may take several days or a week after the last dose is taken for these problems to improve, though.

What are the long-term side effects of anastrozole?

Women typically take daily doses of letrozole for five, seven, or 10 years. Such long-term use is associated with some anastrozole side effects such as weight gain, osteoporosis, hair loss, and bone fractures. These are typically the result of years of estrogen suppression.

Anastrozole contraindications

Anastrozole is never prescribed to women with a known allergy to the drug.

Pregnancy

Anastrozole is not approved for use in women of reproductive age. Anastrozole can poison or harm a fetus, so premenopausal women who are given anastrozole off-label along with ovary suppressing drugs for early breast cancer should avoid pregnancy by using contraception throughout treatment and for up to three weeks after the last dose is taken. Fertility treatments are different. Several doses will be taken around the time of ovulation. No further doses are given if a pregnancy occurs. 

Breastfeeding

Anastrozole is not approved for use in premenopausal women, so there are no studies of the drug in women who are breastfeeding. Because of potential harm to the infant, women should not breastfeed when taking anastrozole.

Children

The Food and Drug Administration (FDA) has not approved anastrozole for use in children or teens. 

Seniors

Most of the participants in clinical trials have been older than 65. There are no special concerns about using anastrozole in seniors.

Anastrozole warnings

Outside of side effects, there aren’t many serious issues concerning anastrozole and other medical conditions, overdosing, or physical dependence.

Cautions

Anastrozole can worsen some pre-existing conditions including heart disease related to narrowed arteries, high cholesterol, and osteoporosis.

Abuse and dependence

Anastrozole is not associated with abuse or physical dependence.

Overdose

If too much anastrozole is taken, contact a healthcare provider or poison control center. There are no known symptoms of an anastrozole overdose, and no life-threatening effects have been reported. 

Anastrozole interactions

There aren’t many problems caused by combining anastrozole with other drugs. However, because anastrozole blocks the synthesis of estrogen, it is never prescribed to women taking estrogens either for birth control or hormone therapy.

For women with hormone receptor-positive breast cancer, the alternative to aromatase inhibitor treatment is tamoxifen, a drug that blocks the effects of estrogen. It is sometimes used for several years before anastrozole therapy, but it is never prescribed along with anastrozole. It doesn’t really help and adds on side effects of its own.

Finally, DHEA supplements, whether prescribed by a healthcare provider or purchased over the counter, should be avoided by anyone taking an estrogen inhibitor. These substances are precursor chemicals to testosterone and estrogen and could reduce the effectiveness of estrogen suppression treatments like anastrozole.

How to avoid anastrozole side effects

Anastrozole has a large number of possible side effects that have a significant incidence rate. That means side effects can and should be expected when taking anastrozole or other estrogen suppression treatments. But the problems don’t have to be bad or frequent. A few smart moves can help minimize or even prevent some side effects.

1. Let the prescriber know about all current medical conditions

In nearly every case, the oncologist prescribing anastrozole has an excellent picture of the person’s current health and medical history. Medical records will have been studied and many imaging and blood tests performed. Still, the prescriber needs to know about all current medical conditions including those that may not be in the medical record. To prevent potentially serious side effects, the oncologist particularly needs to know about:

  • Periods
  • Problems with osteoporosis or low bone density
  • Heart problems
  • Chest pain 
  • High cholesterol
  • Any female hormone replacement treatments, birth control pills, or other hormonal contraception
  • Tamoxifen treatment
  • Pregnancy or pregnancy plans
  • Breastfeeding or breastfeeding plans

2. Keep all follow-up appointments

Throughout treatment—which could last 10 years—make sure to keep all appointments with healthcare providers. They will need to perform regular imaging and blood tests to help spot problems such as bone loss, high cholesterol, and other issues that could evolve into serious medical problems.

3. Follow the instructions

Take this medicine as instructed by the prescriber. If these have been forgotten, they’ll be written on the prescription label on the medicine bottle. 

4. Don’t take double doses to make up for a missed dose

Take only a single 1 mg tablet per day. If the dose is forgotten, it can be taken on the same day. If it’s the next day, forget it. Take the next dose—just one tablet—as scheduled. 

5. Take each day’s dose with a meal or snack

Anastrozole is just as effective whether it’s taken with or without food. However, taking each dose with a meal or snack can help prevent nausea and vomiting.

6. Take calcium supplements

Reducing the estrogen levels in the body causes bone loss, so consider adding daily calcium and vitamin D supplements. Dietary changes can help, too. Foods rich in calcium and vitamin D include milk, cheese, eggs, and fatty fish.

7. Exercise

Weight-bearing exercise helps to build bone mineral density. Do exercises regularly that involve staying on your feet like walking, jogging, running, or stair-climbing. 

8. Eat a healthy diet

Speaking of diet, not only can a healthy diet help prevent osteoporosis, but it also can help prevent fatigue or weight gain.

9. Don’t smoke

Smoking can worsen bone loss and the risk for bone fractures due to low estrogen. The nicotine in cigarettes also can make hot flashes worse.

10. Ease up on drinking

Alcohol can enhance hot flashes, flushing, dizziness, or tiredness caused by anastrozole. Otherwise, alcohol does not cause serious problems when combined with anastrozole.

How to treat side effects of anastrozole

Most people will experience some side effects when taking anastrozole, but there are ways to treat them at home. When there aren’t, it’s time to talk to a healthcare professional.

Hot flashes

Hot flashes, flushing, and night sweats are common problems with estrogen suppressing therapy. Cooling pads, cold drinks, and even spritzing the face with water can help. Dress in layers and be prepared to shed them when a hot flash or flushing occurs. If hot flashes are too hard to live with, talk to the prescriber. Some drugs can help such as SSRIs and SNRIs, antidepressants that have a long track record in the treatment of hot flashes and flushing. 

Nausea and vomiting

Make sure to drink fluids, but only take small sips. An antihistamine might improve nausea. If vomiting is severe or hangs on for a long time, talk to a healthcare professional.

Aches and pain

Anastrozole can cause a bunch of aches and pains throughout the body. Start by moving around. Exercise and stretching often help reduce pain. If that doesn’t help, ask the prescriber about over-the-counter pain relievers. If the pain is severe or doesn’t get better over time, talk to the prescribing healthcare provider.

Osteoporosis

If there are problems with bone mineral density, healthcare providers will use medicines called bisphosphonates to help increase bone density.

Chest pain

Anastrozole can reduce blood flow to the heart, so report any chest pain to the prescribing healthcare provider.

Allergic reactions

Stop taking anastrozole and get medical help at any sign of an allergic or skin reaction including swelling of the face (or throat, mouth, or tongue), shortness of breath, trouble swallowing, rash, skin blisters, skin sores, skin lesions, or peeling skin.

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