Headache, dry mouth, nausea, anxiety and insomnia are the most commonly reported side effects of modafinil. They are usually dose dependent and improve with lower doses, better hydration, earlier timing or taking the tablet with a small amount of food. Headaches in particular often respond to extra water and electrolytes, since modafinil can suppress thirst.
The more serious concern is hypersensitivity. Modafinil carries a warning for severe skin reactions including Stevens-Johnson syndrome, though the absolute incidence is low. Any rash, blistering, mouth sores, fever or facial swelling after a dose should be treated as a stop signal. Do not continue taking the compound and seek medical evaluation if these symptoms appear.
Psychiatric side effects such as agitation, hallucinations or suicidal thoughts have also been reported, usually in people with a history of mood disorders. Modafinil is not a benign productivity hack for everyone. Starting low, avoiding combination with other stimulants and monitoring the first few doses carefully reduces the most common problems.