Possible side effects of taking melatonin include dizziness, daytime sleepiness, headache and short-term feelings of depression, according to WebMD. Some people may feel irritable after taking melatonin, and it may cause stomach cramps. After taking melatonin, it is important to wait at least four to five hours before using machinery or driving a vehicle.
When used short-term orally or topically, melatonin is likely safe for the majority of adults, notes WebMD. It is possibly safe when taken long-term orally in recommended dosages. Melatonin is possibly unsafe for use during pregnancy, and it may reduce a woman's chances of getting pregnant by interfering with ovulation. Because it is not known if melatonin is safe when breast-feeding, nursing women should not take it, as of 2015.
Most children should not use melatonin, as it is possibly unsafe, advises WebMD. Melatonin may exacerbate bleeding disorders and depression, and it may cause blood pressure levels to rise for people who take certain blood pressure medications. People with diabetes should carefully monitor blood glucose levels when taking melatonin, as it may increase blood sugar.
People who are at risk of a seizure should speak to a doctor before using melatonin, as it may increase the risk, states WebMD. Melatonin may increase immune function, which can cause complications for people receiving transplants.