Letrozole is a tablet taken once daily by mouth, with or without food. How you use it depends on why it was prescribed: a short 5-day course for ovulation induction during fertility treatment, or a daily regimen lasting several years for hormone-receptor-positive breast cancer. Both uses rely on the same core action, but the dosing schedules, monitoring, and goals look very different.
How Letrozole Works in the Body
Letrozole blocks an enzyme called aromatase, which your body uses to produce estrogen. By temporarily shutting down estrogen production, the drug triggers a chain reaction. Your brain detects the drop in estrogen and responds by releasing more follicle-stimulating hormone (FSH), which pushes your ovaries to develop and release eggs. This is why it works for fertility.
In breast cancer, the logic flips. Many breast tumors depend on estrogen to grow. By keeping estrogen levels consistently low over months or years, letrozole starves those tumors of the hormone they need.
Using Letrozole for Fertility
For ovulation induction, letrozole is taken for five consecutive days early in your menstrual cycle, typically starting on cycle day 3 (counting the first day of your period as day 1). The standard starting dose is 2.5 mg per day, though your doctor may prescribe up to 7.5 mg daily depending on how your body responds.
Take your tablet at roughly the same time each day. You can take it with or without food, and you don’t need to change your normal diet. If you miss a dose during the 5-day window, take it as soon as you remember. Since the course is only five days, consistency matters.
After you finish the five tablets, your body does the rest. The temporary estrogen drop has already signaled your brain to release FSH, and your ovaries begin growing follicles. Around cycle day 13, you’ll typically have a mid-cycle ultrasound to check follicle development. The goal is one to three follicles measuring 16 mm or larger. If a dominant follicle reaches 18 mm or more, your doctor may give you an injection to trigger ovulation, after which you’ll be advised to time intercourse or undergo intrauterine insemination within 24 to 36 hours.
Most fertility protocols involve trying letrozole for up to three to six cycles before reassessing your treatment plan. Not every cycle results in pregnancy, but letrozole has strong evidence behind it. A meta-analysis comparing letrozole to clomiphene citrate (the older standard) in women with polycystic ovary syndrome found letrozole produced a 54% higher live birth rate. It also carried a lower risk of twins or triplets. These results are a major reason letrozole is now considered the first-line fertility medication for PCOS.
Using Letrozole for Breast Cancer
For postmenopausal breast cancer, letrozole is taken as a single 2.5 mg tablet every day for years. The duration depends on your treatment history. Some protocols call for two to three years of letrozole after an initial course of tamoxifen, while extended regimens run for a full five years. Research on sequential therapy found that tamoxifen for two to three years followed by letrozole for five years is one of the most effective endocrine treatment strategies available.
Because you’re taking it long-term, building a daily habit matters more here. Pick a consistent time, whether that’s with breakfast or before bed, and stick with it. You don’t need to take it with food, but pairing it with an existing routine makes it easier to remember.
What Side Effects to Expect
In clinical data, roughly half of patients on letrozole reported noticeable side effects within their first few visits. The most common, by a wide margin, is joint pain, affecting about 30% of users. Hot flashes come next at 25%, followed by new or worsening fatigue at 16%. Muscle stiffness and muscle pain round out the list at roughly 8% and 7%, respectively.
For short-term fertility users, these side effects are generally mild and resolve quickly after the 5-day course ends. Long-term users being treated for breast cancer are more likely to experience persistent joint pain and stiffness, which is one of the main reasons some patients consider stopping treatment early. If joint symptoms become disruptive, your care team can often adjust the approach rather than discontinuing treatment entirely.
Letrozole can also cause dizziness in some people, so it’s worth seeing how it affects you before driving or operating machinery.
Important Safety Information
Letrozole is strictly contraindicated during pregnancy and breastfeeding. This sounds counterintuitive for a fertility drug, but the distinction matters: you take letrozole before ovulation to help you conceive, then stop. If you become pregnant, you should not continue taking it. Case reports and animal studies show potential for fetal harm from ongoing exposure.
Pregnant women should also avoid handling letrozole tablets or breathing in dust from broken tablets, since the drug can be absorbed through the skin and lungs.
What Monitoring Looks Like
If you’re using letrozole for fertility, expect at least two clinic visits per cycle. The first is baseline testing on cycle day 2 through 5, before you start the medication. This includes an ultrasound and blood work to confirm you’re not pregnant and that your ovaries are at a resting state. The second visit is the mid-cycle ultrasound around day 13 to measure follicle growth and decide on timing.
Some cycles require additional ultrasounds if follicles are growing slowly or if your doctor wants to track ovulation more precisely. Daily ultrasound monitoring may continue until ovulation is confirmed, or every two to three days if ovulation hasn’t occurred within the expected window.
For breast cancer patients, monitoring is less frequent but still ongoing. Your oncologist will check in at regular intervals to assess side effects, bone density (since low estrogen can weaken bones over time), and overall treatment response.
Tips for Getting the Most From Your Cycle
If you’re using letrozole for fertility, a few practical habits can help. Take the tablet at the same time each day during your 5-day course to keep hormone levels consistent. Keep track of your cycle day so you don’t miss your monitoring appointment. If your clinic uses a trigger shot to induce ovulation, follow the timing instructions precisely, as even a few hours can shift your fertile window.
If a cycle doesn’t work, that’s common and expected. Your doctor may increase the dose on the next cycle (from 2.5 mg to 5 mg, for example) or add additional monitoring. The process is iterative, with each cycle providing information that helps fine-tune the next one.