Letrozole: what it treats, how to take it, side effects
Last updated September 3, 2026.
Letrozole is an aromatase inhibitor - it decreases the amount of estrogen the body produces, slowing or stopping breast cancer cells that need estrogen to grow. You may know it as Femara. It is a long-game medication: years of one daily tablet standing between remission and recurrence.
What does it treat?
Hormone-sensitive breast cancer in women after menopause: early cancer after surgery or radiation, extended treatment after 5 years of tamoxifen, and cancer that has spread or worsened on tamoxifen. It starves the cancer's fuel supply rather than attacking the cells.
How do you take it?
One tablet once a day, with or without food, same time every day - for several years or longer. Continue even when you feel well; feeling well is the drug working. Never stop without your oncologist. Missed dose: take it when you remember unless the next one is close; never double up.
Side effects to know
- Common: hot flushes and night sweats, nausea, vomiting, appetite loss, constipation, diarrhea, heartburn, stomach pain, weight changes, muscle, joint, or bone pain, excessive tiredness, headache, dizziness, weakness, swelling of the hands, feet, ankles, or lower legs, sleep trouble, and vaginal bleeding or irritation. The joint and bone pain is the signature complaint - report it, because it is manageable.
- Serious - call your doctor immediately or get emergency care: chest pain; rash, hives, or itching; difficulty breathing; unusual bleeding or bruising; pain in the upper right part of the stomach; and yellowing of the skin or eyes - the liver signal.
Who should not take it?
It is built for postmenopausal bodies - women who have not reached menopause are prescribed it differently, with ovarian suppression, if at all. Flag liver disease, osteoporosis, and high cholesterol before starting: estrogen suppression thins bone over the years, and bone-density checks belong in the plan. If there is any chance of pregnancy, say so immediately.
When is it an emergency?
Chest pain, trouble breathing, or yellowing skin or eyes is urgent care. For an overdose, call Poison Control at 1-800-222-1222. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
Why do I take letrozole for years if the cancer is gone?
Because hormone-sensitive breast cancer can recur years later, and the estrogen suppression is the fence. The evidence behind 5 to 10 years of aromatase-inhibitor therapy is why MedlinePlus says you may need it for several years or longer and to continue even if you feel well. Every year on it is doing quiet work you cannot feel - that is the design.
Why do my joints ache on letrozole?
Estrogen protects joints and bones - remove it and the ache arrives. Muscle, joint, and bone pain are on the common-effects list, and for many they peak in the first year then ease. Report it rather than endure it: vitamin D checks, exercise, pain strategies, and occasionally a switch within the aromatase family are all on the table. The ache is a reason to adjust the plan, not abandon it.
Why does my oncologist check my bones and cholesterol?
Long-term estrogen suppression thins bone - osteoporosis risk climbs over the years on letrozole - and cholesterol can drift up. The monitoring is built into the therapy: bone-density scans, vitamin D and calcium, weight-bearing exercise, and lipid checks. The drug protects you from the cancer; the monitoring protects you from the drug.
How is letrozole different from tamoxifen?
Both fight estrogen-driven breast cancer, opposite tactics: tamoxifen blocks the estrogen receptor on the cancer cell; letrozole shuts down estrogen production itself. After menopause, aromatase inhibitors like letrozole tend to edge out tamoxifen - which is why they are first-line there, and why a 5-year tamoxifen course often transitions into letrozole for extended protection.
Why did the team ask if I was sure I was postmenopausal?
Aromatase inhibitors only suppress estrogen made outside the ovaries - working ovaries will simply make more. In a premenopausal woman, letrozole alone fails and can even stimulate. That is why the menopause confirmation is not a formality, and why any sign of returning periods or pregnancy on the drug is an immediate call.
What happens if I miss a dose of letrozole?
Take it as soon as you remember, unless it is almost time for the next one - then skip it and never double up. A years-long course absorbs single misses without blinking; the habit that matters is the same-time-daily anchor, which is what keeps misses rare.