Modafinil is a wakefulness enhancing agent approved for sale by the FDA. Modafinil has been in use around the world for over 25 years. In 2001 alone it was prescribed to over 1 million patients in the United States. It is a closely related to Armodafinil (also known as Nuvigil® or Waklert).
Modafinil itself is based on an older medication developed in the 1970s known as Adrafinil. Adrafinil is metabolized by the body into Modrafinil, and is much less effective. Use of Adrafinil cannot be recommended, due to its side effects of stomach pain, skin irritations, feelings of tension, and increases in liver enzyme levels. It is also very important to monitor liver function in Adrafinil users, as it has been known to cause liver damage. Neither Modafinil or Armodafinil are known to have an affect on liver function. Needless to say, Modafinil is a lot safer than Adrafinil.
Most people using Modafinil for performance do not use it every day, or even every week. Instead they make use of it on days when they need an extra boost in energy, concentration or creativity.