Most Common Effect

• In clinical trials, the most common side effect associated with modafinil was headaches, occurring in up to 34 percent of participants.
  
  

Other Common Effects

• Other side effects occurring between 5 and 11 percent of people included back pain, diarrhea, dizziness, indigestion or heartburn, insomnia, nausea, nervousness, and runny or stuffy nose.
  
  

Less Common Effects

• Many other less common side effects are possible. These include constipation, drowsiness, dry mouth, flushing or hot flashes, increased perspiration, loss of appetite, neck stiffness, nosebleeds, shakiness or tremors, and sore throat.
  
  

Serious Side Effects

• Rare but serious side effects should be addressed by a doctor. These include blood in the urine, hallucinations, chest pain, heart palpitations, high blood pressure and unusual muscle spasms.
  
  

Warning

• Modafinil can cause a rare and extremely serious side effect called Stevens-Johnson syndrome (SJS). This disorder begins as a painful rash and can progress to loss of skin, which can become life-threatening.