Melatonin is a hormone that helps maintain the body's natural clock, or circadian rhythm. This body clock influences when people fall asleep and wake up, so melatonin supplements are often used to treat sleep disorders and conditions such as jet lag. Other potential uses include treating attention deficit hyperactivity disorder, cancer and glaucoma, although studies have not proven its efficacy in treating these conditions. For most people, melatonin is well tolerated, but side effects are possible.
Fatigue
One side effect commonly attributed to melatonin is fatigue and sleepiness. However, in the case of people taking the supplement to combat jet leg, these side effects may be due more to the jet lag itself than melatonin, according to the National Institutes of Health.
Mood Changes
Significant mood changes, including extreme sadness and giddiness, have also been reported as side effects of melatonin. Some people also notice they are more irritable when using the supplement.
Sleep Effects
Some people taking melatonin have reported experiencing vivid dreams and nightmares, notes the National Institutes of Health. Sleepwalking is also possible, as well as disturbances to the sleep cycle because of melatonin's effect on the body's circadian rhythms.
Sexual Side Effects
In men, melatonin can lead to breast enlargement and may also decrease sperm count, according to the UMMC. A woman's fertility may also be affected by the supplement, and it should therefore not be used by women who are pregnant or breastfeeding.
Seizures
The National Institutes of Health explain that study results have been mixed with regard to melatonin's ability to prevent seizures, with some studies showing it increases the risk of seizure, while others show that it lowers it. Because this remains unknown, people with seizure disorders should only use melatonin under the close supervision of a doctor.
Depression
Some evidence has shown that taking melatonin can cause depression symptoms to worsen in people who have depression, reports the UMMC.
Stomach Upset
Another possible side effect of melatonin is nausea or an upset stomach, according to Drugs.com. Some people also develop diarrhea and abdominal cramping.
Blood Clotting
People with blood clotting disorders should use caution if taking melatonin because it may interfere with the body's ability to clot. In particular, people taking the drug warfarin and other blood thinners should avoid using melatonin, advises the National Institutes of Health.
Blood Pressure
Use of melatonin supplements may also cause blood pressure to drop. Because of this, people taking blood-pressure-lowering drugs should not take melatonin, notes the National Institutes of Health. In some cases, abnormal heart rhythms have also been linked to melatonin.
Cholesterol
The National Institutes of Health reports that evidence from studies suggests that melatonin may cause an increase in cholesterol levels. Therefore, the NIH does not recommend the use of the supplement by people with high cholesterol or atherosclerosis and those at risk for heart disease.
Blood Sugar
People with type 1 diabetes have in some cases developed hyperglycemia--a high blood sugar level--when taking melatonin. People with diabetes or other conditions related to blood sugar levels should use caution when taking the supplement, advises the National Institutes of Health.
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