Modafinil is generally well-tolerated but can have potential risks and side effects. Common adverse effects of modafinil, experienced by less than 10% of users, include headaches, nausea, and reduced appetite. Anxiety, insomnia, dizziness, diarrhea, and rhinitis are also reported in 5% to 10% of users. Psychiatric reactions have occurred in individuals with and without a preexisting psychiatric history.
No significant changes in body weight have been observed in clinical trials, although decreased appetite and weight loss have been noted in children and adolescents. Modafinil can cause a slight increase in aminotransferase enzymes, indicative of liver function, but there is no evidence of serious liver damage when levels are within reference ranges.
Rare but serious adverse effects include severe skin rashes and allergy-related symptoms. Between December 1998 and January 2007, the FDA received reports of six cases of severe cutaneous adverse reactions, including erythema multiforme, Stevens–Johnson syndrome, toxic epidermal necrolysis, and DRESS syndrome. The FDA has issued alerts regarding these risks and also noted reports of angioedema and multi-organ hypersensitivity reactions in postmarketing surveillance. In 2007, the FDA required Cephalon to modify the Provigil leaflet to include warnings about these serious conditions. The long-term safety and effectiveness of modafinil have not been conclusively established.
The FDA does not endorse modafinil for children's medical conditions due to an increased risk of rare but serious dermatological toxicity (Stevens–Johnson syndrome). However, in Europe, modafinil may be prescribed for treating narcolepsy in children.