When treating any condition, finding effective treatments involves weighing the benefits and potential 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 level of estrogen, an important sex hormone. Reducing estrogen levels can result in a variety of potential side effects, including the dozen discussed below.

1. Metallic Taste

Rarely, using letrozole may result in changes in your sense of taste, including a persistent 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.

2. Symptoms of Menopause

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 study findings published in the Journal of Clinical Oncology, around one-third of women taking letrozole reported experiencing 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!”

3. Sexual Side Effects

Some of the menopauselike side effects of letrozole can cause vaginal problems such as dryness, bleeding, or increased discharge. Several MyBCTeam members have mentioned experiencing related sexual side effects. One member discussed vaginal 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!”

4. Aches and Pains

One of the most common side effects of taking letrozole is back pain, as well as pain in 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!”

5. Sleep Problems

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

6. Thinning of Bones

As with menopause, a lack of estrogen caused by letrozole can affect the amount of calcium stored in your bones, leading to bone thinning. In the early stages, this loss in bone density is known as osteopenia, which can progress to osteoporosis (brittle bones). Nearly 11 percent of women taking letrozole develop osteoporosis, according to the Journal of Clinical Oncology study. Thinning bones can increase your risk of fractures, so your doctor will closely monitor your bone density while you’re on letrozole.

7. Dry Skin and Hair Loss

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

In addition, letrozole may cause dry skin, leading to irritation, itchiness, or swelling. 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.”

8. Digestive Tract Issues

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.”

9. Anxiety and Depression

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.”

10. Flu-Like Symptoms

Fu-like symptoms can indicate that you have an infection and should seek medical help, but letrozole can 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.”

11. Weight and Appetite Changes

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 could also cause a loss of appetite and weight. 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.”

12. More Serious Side Effects

It's important to be aware of some rare but serious side effects of letrozole. Notify your health care provider immediately to get medical advice if you experience any of the following:

• Signs of a blood clot, like swelling or pain along a vein
• 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 for emergency care if you think you’re having a stroke or heart attack.

Your Health Care 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 might need to try a different breast cancer treatment. It’s important to learn about the possible side effects of new medications so that you know what to expect and can discuss any concerns with your doctor.

Talk With Others Who Understand

MyBCTeam is the social network for people with breast cancer and their loved ones. On MyBCTeam, more than 64,000 members come together to ask questions, give advice, and share their stories with others who understand life with breast cancer.

Have you noticed any of these side effects while taking letrozole? Share your experience in the comments below, or start a conversation by posting on your Activities page.