Considering the long shelf life of most supplements and the safety of melatonin, using expired melatonin may not be harmful. However, you might notice that expired melatonin doesn't make you as tired or drowsy as unexpired melatonin.

How long is melatonin good for?

Melatonin has been used safely for up to 2 years in some people. However, it can cause some side effects including headache, short-term feelings of depression, daytime sleepiness, dizziness, stomach cramps, and irritability. Do not drive or use machinery for four to five hours after taking melatonin.

Why is melatonin bad?

While short-term use of melatonin in adults is generally considered safe, taking too much can lead to bad dreams and grogginess the next day, notes Breus. It can also make some drugs less effective, including high blood pressure medications and, potentially, birth control pills.