Letrozole belongs to the group of cancer-fighting medications known as antineoplastics, and specifically to the type of antineoplastics known as nonsteroidal aromatase inhibitors.

It is very important that this medication be taken exactly as prescribed by your doctor.

The following side effects have been reported by at least 1% of people taking this medication.
constipation
diarrhea
dizziness
fatigue
hair loss
headache
hot flushes
increase or loss of appetite
increased sweating
indigestion
nausea
pain in muscles, bones, or joints
rash
swelling or puffiness due to retained body fluid
weight increase
vomiting

Check with your doctor immediately if you experience unusual vaginal bleeding

Stop taking the medication and seek immediate medical attention if any of the following occur:
Signs of a blood clot in the arm or leg (tenderness, pain, swelling, warmth, or redness in the arm or leg) or lungs (difficulty breathing, sharp chest pain that is worst when breathing in, coughing, coughing up blood, sweating, or passing out)

Any estrogen-containing medications may affect the way letrozole works or increase the risk of side effects.

In case of overdose, call your local poison control center or emergency services. If the victim has collapsed or is not breathing, call local emergency services at once.

Letrozole should not be used by anyone who is:

allergic to any of the ingredients
is allergic to any other aromatase inhibitor medications (e.g., anastrozole, exemestane)
is breast-feeding
pregnant
premenopausal

If you miss a dose, take it as soon as possible and continue with your regular schedule. If it is almost time for your next dose, skip the missed dose and continue with your regular dosing schedule. Do not take a double dose to make up for a missed one.

Letrozole can be used to treat advanced breast cancer in postmenopausal women (women who are past menopause), including those who are no longer getting the beneficial effects of anti-estrogen medications such as tamoxifen. Letrozole can also reduce the risk of breast cancer recurrence in postmenopausal women who have been successfully treated for early stage breast cancer and who have completed five years of tamoxifen therapy to prevent recurrence.

Your doctor may have suggested this medication for conditions other than the ones listed in these drug information articles. If you have not discussed this with your doctor or are not sure why you are taking this medication, speak to your doctor. Do not stop taking this medication without consulting your doctor.

Store the tablets in a dry place at room temperature.

Be sure to inform your doctor of all your medical conditions before you begin taking a medication. Some conditions can affect how you should take this medication.
Long-term use of letrozole may decrease the density of bones, increasing the risk of osteoporosis.
Letrozole may increase cholesterol levels.
Letrozole may cause dizziness or drowsiness.
Letrozole may cause kidney function impairment.
People with severe liver disease who take this medication should be closely monitored by their doctor.
Letrozole should not be used by women who have not reached menopause unless the potential benefits outweigh the risks.
Letrozole has not been studied for use by pregnant women, since it is intended only for women past menopause. However, it is suspected that its use during pregnancy could cause miscarriages and other serious problems. Therefore, it should not be taken during pregnancy.
This medication is not recommended for children.