The U.S. Food and Drug Administration does not strictly regulate herbs and supplements. There is no guarantee of strength, purity or safety of products, and effects may vary. You should always read product labels. If you have a medical condition, or are taking other drugs, herbs, or supplements, you should speak with a qualified healthcare provider before starting a new therapy. Consult a healthcare provider immediately if you experience side effects.
Avoid in those with known allergies to melatonin or related products. There are rare reports of allergic skin reactions after taking melatonin by mouth. Melatonin has been linked to a case of autoimmune hepatitis.
Side Effects and Warnings
Based on available studies and clinical use, melatonin is generally regarded as safe in recommended doses (five milligrams daily) for up to two years. Available trials report that overall adverse effects are not significantly more common with melatonin than placebo.
Commonly reported adverse effects include fatigue, dizziness, headache, irritability, drowsiness, and sleepiness, although these effects may occur due to jet lag and not due to melatonin itself. Fatigue may occur particularly with morning use or high doses, and irregular sleep-wake cycles may occur. Disorientation, ataxia (difficulties with walking and balance), confusion, sleepwalking, vivid dreams, and nightmares have also been noted, with effects often resolving after the cessation of melatonin. Due to the risk of drowsiness or sedation, use caution if driving or operating heavy machinery.
Mild gastrointestinal distress commonly occurs, including nausea, vomiting, cramping, stomach pain, altered taste, abnormal feces, increased appetite, or diarrhea. Melatonin has been linked to the triggering of Crohn's disease symptoms.
Other side effects include skin rash, infection, decreased mental performance, increased risk of inflammation, and increased urination.
It has been suggested that melatonin may lower the seizure threshold and increase the risk of seizure, particularly in children with severe neurologic disorders. However, multiple other studies actually report reduced incidence of seizure with regular melatonin use. This remains an area of controversy. Patients with seizure disorder taking melatonin should be monitored closely by a healthcare professional.
Mood changes have been reported, including giddiness and dysphoria (sadness). Psychotic symptoms have been reported, including hallucinations and paranoia, possibly due to overdose. Patients with underlying major depression or psychotic disorders taking melatonin should be monitored closely by a healthcare professional.
Melatonin may increase the risk of bleeding. Caution is advised in patients with bleeding disorders or those taking drugs that may increase the risk of bleeding. Dosing adjustments may be necessary.
Melatonin may cause drops in blood pressure. Caution is advised in patients taking medications that may also lower blood pressure.
Based on preliminary evidence, increases in cholesterol levels may occur. Caution is therefore advised in patients with high cholesterol levels or atherosclerosis, or in those at risk for cardiovascular disease. Abnormal heart rhythms have been associated with melatonin.
Hyperglycemia (elevated blood sugar levels) has been reported in patients with type 1 diabetes (insulin-dependent diabetes), and low doses of melatonin have reduced glucose tolerance and insulin sensitivity. Caution is advised in patients with diabetes or hypoglycemia, and in those taking drugs, herbs, or supplements that affect blood sugar. Serum glucose levels may need to be monitored by a healthcare provider, and medication adjustments may be necessary.
Hormonal effects have been reported, including decreases or increases in levels of luteinizing hormone, progesterone, estradiol, thyroid hormone (T4 and T3), growth hormone, prolactin, cortisol, oxytocin, and vasopressin. Gynecomastia (increased breast size) has been reported in men, as well as decreased sperm count (both which resolved with the cessation of melatonin). Decreased sperm motility and decreased libido have been reported in rats and humans.
It has been theorized that high doses of melatonin may increase intraocular pressure and the risk of glaucoma, age-related maculopathy and myopia, or retinal damage. However, there is preliminary evidence that melatonin may actually decrease intraocular pressure in the eye, and it has been suggested as a possible therapy for glaucoma. Patients with glaucoma taking melatonin should be monitored by a healthcare professional.
Use cautiously in patients using anesthetics, cytochrome P450 1A2 inhibitors like fluvoxamine, methamphetamine, nifedipine (a calcium channel blocker), or in children with a history of enuresis.
Avoid in patients using CNS depressants, including benzodiazepines and alcohol, as concomitant use may cause increased sedation.
Avoid use in women who are pregnant or are attempting to become pregnant, based on possible hormonal effects and a possible increased risk of developmental disorders.
Avoid with known allergies to melatonin or related products.
Pregnancy and Breastfeeding
Melatonin supplementation should be avoided in women who are pregnant or attempting to become pregnant, based on possible hormonal effects. High levels of melatonin during pregnancy may increase the risk of developmental disorders. In animal studies, melatonin was detected in breast milk and therefore should be avoided during breastfeeding. In men, decreased sperm motility and decreased sperm count are reported with the use of melatonin.