When treating any condition, finding the right treatment involves weighing the potential benefits and side effects of medications. For example, letrozole (Femara), a hormonal therapy typically used for breast cancer, can lead to side effects ranging from an annoying metallic taste to serious health conditions.

Letrozole works by lowering your body’s levels of estrogen, an important sex hormone. Reducing estrogen levels can result in a variety of side effects, including those discussed below.

Rarely, using letrozole may result in changes in your sense of taste, including a metallic taste in your mouth, medically known as dysgeusia.

Letrozole can also cause dry mouth, which can contribute to changes in how you experience flavors, including a tang that tastes like you have metal in your mouth.

Because it blocks estrogen production, letrozole can cause symptoms similar to those associated with menopause, such as hot flashes, vaginal dryness, and mood changes.

In clinical trials of letrozole in postmenopausal women with hormone receptor-positive breast cancer, about one-third reported hot flashes.

Several MyBCTeam members have also described this side effect in comments like these:

“My body feels like an oven warming up”

“I’m not having hot flashes. … I’m having nuclear meltdowns!”

Some of the side effects of letrozole that resemble symptoms of menopause can cause vaginal problems such as dryness, bleeding, or increased discharge. Several MyBCTeam members have mentioned experiencing related sexual side effects.

One member discussed using vaginal moisturizer for dryness: “I use Replens consistently, and it still feels like razor blades and like the walls are so close that I can feel the sting even when I sit. Terrible at 60 to think this will be the way I have to live my life — with painful sex.”

Another member expressed a desire to regain their quality of life: “I don’t want to be limited — I want my life back.”

In response, a member emphasized the importance of advocating for yourself: “Remember, your happiness and pleasure matter!”

One of the most common side effects of taking letrozole is back pain, as well as pain in the bones, joints, and muscles throughout the body. In the clinical study mentioned above, almost half of the participants who received letrozole reported experiencing joint pain.

Although this side effect may linger as long as you take letrozole, some MyBCTeam members noticed improvement in their symptoms over time.

“For the first six months, I had some pretty significant leg and hip pain, especially if I sat too long,” said one member. “After six months, the pain started getting better, and now I hardly have any pain!”

“For the first six months, I had some pretty significant leg and hip pain, especially if I sat too long.”

— A MyBCTeam member

Another possible side effect of letrozole is trouble sleeping. If you have difficulty sleeping most nights, you may have insomnia.

One MyBCTeam member shared their experience: “I don’t sleep more than four to five hours a night.”

Letrozole may lead to tiredness that makes the lack of sleep feel even worse. “The fatigue sets in around 2 p.m. to 3 p.m.,” the same member said. “I rest but can’t take a nap; I just can’t fall asleep!”

Bone pain is a common side effect of letrozole. The medication can decrease bone mineral density and raise the risk of osteopenia (loss of bone density) and osteoporosis (weak, brittle bones).

Exactly how often osteoporosis happens can vary by study and by a person’s risk factors. Thinning bones can increase your risk of fractures, so your healthcare provider may monitor your bone density while you’re on letrozole.

Your healthcare provider may recommend a baseline bone density scan called dual-energy X-ray absorptiometry (DEXA) and repeat scans over time.

They may also suggest steps to protect your bones, such as:

• Doing weight-bearing exercise
• Getting enough calcium and vitamin D
• Taking bone-strengthening medication like zoledronic acid or denosumab (typically recommended only for people at higher risk)

You may notice changes in your skin and hair while on letrozole. Alopecia (hair thinning or hair loss) affected about 6 percent of women taking letrozole according to the study mentioned above.

This side effect can be upsetting, especially if you’ve recently regrown your hair after finishing chemotherapy. “I can handle the aches and pains, but the extreme hair loss is the most frustrating side effect for me,” a MyBCTeam member said.

“I can handle the aches and pains, but the extreme hair loss is the most frustrating side effect for me.”

— A MyBCTeam member

In addition, letrozole may cause dry skin, skin rash, or itchiness.

One member reflected on switching from anastrozole (Arimidex) to letrozole: “I have noticed significant aging effects like hair loss, dry skin, loss of muscle tone, very dry vaginal fluid, dry eyes, more wrinkles, and more fatigue.”

Letrozole can upset your stomach, causing nausea, stomach pain, diarrhea, or constipation.

“I’ve had hard and painful bowel movements,” wrote a MyBCTeam member who developed constipation as a side effect. “I tried everything and still only had one every two days. I finally started eating fresh fruit and found that cantaloupe works well for me.”

Letrozole can affect your mental health, making you feel anxious or depressed. One MyBCTeam member described feeling sad all the time while taking letrozole: “When I took Femara, all I did was cry. When I slept, my eyes would fill with tears.”

Another member asked, “Has anyone felt a mood drop taking letrozole? I have been taking it since January, and I just don’t feel like doing anything.”

Flu-like symptoms can indicate that you have an infection and should seek medical help, but letrozole can also cause similar side effects, including:

• Chills
• Fever
• Sore throat
• Cough
• Extreme weakness

In the clinical study, about 5 percent of participants taking letrozole developed a cough.

“I’ve been having a dry, hacking cough, and I read that it could be a side effect,” one MyBCTeam member said. “It wakes me up at night and can happen anytime — I can’t stop it — I’ve tried drinking something, cough drops, etc. It isn’t like a cold, so I don’t know what this is.”

If you’ve noticed that your weight fluctuates while taking letrozole, the changes might be related to an increased appetite that leads to weight gain. “I crave doughnuts and pastry like I’m pregnant,” a MyBCTeam member commented.

On the other hand, letrozole may also cause a loss of appetite and weight loss. One member seemed to experience both types of weight-related side effects: “I gained weight with letrozole and then lost it kind of suddenly. It was weird.”

It’s important to be aware of some rare but serious side effects of letrozole for breast cancer. Letrozole can affect things your healthcare provider may monitor with labs or follow-up visits, even if you don’t feel symptoms right away.

Notify your healthcare provider right away to get medical advice if you experience any of the following:

• Persistent fever or chills, which could be a sign of infection
• Signs of a blood clot, such as pain, warmth, or swelling in your leg or arm
• Urinary tract infection symptoms, such as more frequent or painful urination or bloody urine
• Yellowing of the skin or eyes, which may indicate liver problems
• Signs of an allergic reaction, such as an itchy, widespread rash or swelling of the lips, mouth, or throat that makes breathing difficult
• Stroke symptoms, including sudden weakness or numbness, difficulty speaking, loss of coordination, or facial drooping on one side
• Symptoms of a heart attack, such as chest pain or pressure, sudden nausea or vomiting, or pain in the arms, back, neck, jaw, or stomach

Call 911 or seek emergency care if you think you’re having a stroke or heart attack.

Additionally, you should not become pregnant while taking letrozole. The medication can cause serious harm to your baby if you become pregnant during treatment or within three weeks of your last dose. You should also not breastfeed until you have stopped taking letrozole for three weeks.

Your Healthcare Providers Are Your Best Resource

Not everyone taking letrozole will have the same side effects or experience them to the same degree. Some people may have only mild side effects, while others may need to try a different breast cancer treatment.

Learning about possible side effects can help you know what to expect and discuss any concerns with your healthcare provider.

Join the Conversation

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

Have you experienced letrozole side effects? Let others know in the comments below.