When to take letrozole depends on why you’re taking it. For fertility, you’ll take it on specific days of your menstrual cycle, typically starting on cycle day 3 or cycle day 5. For breast cancer, you’ll take it once daily at whatever time fits your routine, continuing for years. Both uses involve the same drug at the same dose (2.5 mg), but the timing rules are completely different.
Timing for Fertility Treatment
If you’re taking letrozole to help you ovulate, the standard protocol is one pill daily for five consecutive days during the early part of your menstrual cycle. Your doctor will tell you to start on either cycle day 3 (three days after your period begins) or cycle day 5. Day 1 is the first day of your period, or the first day of bleeding after progesterone withdrawal if your cycles are irregular.
The start day matters more than you might expect. A study of 539 women with PCOS found that starting on cycle day 5 led to higher ovulation rates (71% of cycles versus 61% for day 3 starts) and higher cumulative conception rates (76% versus 65%). Women in the day 5 group also developed thicker uterine linings, which likely contributed to the better pregnancy outcomes. Starting two days later gives the body a bit more time in the natural hormonal sequence before the medication kicks in, which appears to improve how follicles respond to stimulation.
That said, many clinics still use a day 3 start and get good results. Your provider will choose the timing based on your specific situation, including whether you’re also using other treatments like injectable hormones or timed intercourse.
What Happens After the Five Days
Once you finish the five-day course, your body does the rest. The temporary drop in estrogen caused by letrozole triggers your brain to release more follicle-stimulating hormone, which nudges your ovaries to mature an egg. Most women ovulate roughly 5 to 10 days after their last pill, putting ovulation somewhere around cycle day 14 to 19 depending on when you started. Your clinic will schedule an ultrasound after you finish the medication to check follicle growth and uterine lining thickness, then guide you on when to have intercourse or proceed with insemination.
The starting dose is 2.5 mg daily. If you don’t respond in the first cycle, your doctor may increase to 5 mg or 7.5 mg in a subsequent cycle. Most women who are going to ovulate will do so at the 2.5 mg dose. Higher doses can produce more follicles, but they don’t consistently improve live birth rates and slightly raise the chance of twins.
Timing for Breast Cancer Treatment
For hormone receptor-positive breast cancer, letrozole works very differently. Instead of a short five-day course, you take 2.5 mg every single day for years. Treatment typically begins shortly after surgery, and it can be started before, during, or after radiation therapy. A phase 2 trial confirmed that letrozole can be safely delivered soon after surgery and given at the same time as radiation, so there’s no need to wait until radiation is finished.
The standard treatment length is five years. However, extending to 10 years provides additional benefit for certain patients. A large analysis found that 10 years of hormonal therapy reduced the risk of cancer recurrence by 16% overall compared to five years, and by 30% in postmenopausal women specifically. The patients who benefit most from the longer course tend to be those who are postmenopausal, have hormone receptor-positive tumors, or had cancer that had spread to lymph nodes. Your oncologist will weigh these factors when recommending how long you should stay on treatment.
Morning vs. Evening Dosing
There is no clinical evidence that letrozole works better at one time of day versus another. The pill can be taken morning, afternoon, or night. What matters most is consistency: pick a time you’ll remember every day and stick with it.
Side effects, though, can help you decide. If letrozole makes you feel tired, dizzy, or nauseous, taking it in the evening lets you sleep through the worst of it. If it disrupts your sleep or causes insomnia, switching to a morning dose often helps. You can change the time of day without any special instructions, just shift it at your next dose. Food doesn’t meaningfully affect how well the drug is absorbed. A study in healthy volunteers found no significant difference in total absorption between fed and fasting conditions. The drug has a long half-life of about two days, so a slight delay in absorption from eating makes no practical difference.
If You Miss a Dose
For daily breast cancer dosing, take the missed pill as soon as you remember. If it’s already close to the time for your next dose, skip the missed one entirely and return to your normal schedule. Do not double up. Because letrozole stays in your system for roughly two days, a single missed dose won’t cause a dramatic drop in your drug levels, but making a habit of skipping doses will reduce the medication’s effectiveness over time.
For fertility patients taking a five-day course, a missed dose is more urgent since the timing window is narrow and each pill contributes to that cycle’s ovulation response. If you realize the same day, take it as soon as you can. If you don’t remember until the next day, contact your clinic for guidance on whether to adjust the schedule or continue as planned.