If you’re on steroids, you should avoid melatonin.
Precautions: There are no known serious side effects to regulated melatonin supplementation. Some people may experience vivid dreams or nightmares. Overuse or incorrect use of melatonin could disrupt circadian rhythms. Long-term effects have not been well studied. In rats, melatonin decreases T4 and T3 uptake levels.
Melatonin can cause drowsiness if taken during the day. If morning drowsiness is experienced after taking melatonin at night, reduce dosage levels. In some cases of depression, daytime doses of melatonin can increase depression.
May be contraindicated for those with autoimmune disorders and immune system cancers (e.g., lymphoma, leukemia).
Because melatonin suppresses corticosteroid activity, those who are taking corticosteroids for anti-inflammatory or immune suppressive purposes (e.g., transplant patients) should exercise caution with melatonin supplementation.
Melatonin could interfere with fertility. It is also contraindicated during pregnancy and lactation.
Lack of sleep and insufficient exposure to darkness may suppress natural production of melatonin.
http://www.wellfx.com/InfoBase/vitamin_melatonin_.htm