narcolepsy
"I was given Provigil for my narcolepsy, with dosage of 200mg/3 times daily.  For the first 3 days it was amazing I had forgotten what it felt like to be 'awake'. Unfortunately, after just 3 days it was not effective at all.  My neurologist nodded knowingly when I told him.  He said that's how long it takes to metabolize.  Apparently that's when you can tell if it will work for you or not.  I'm now on Aderall, 20 mg/3 daily and it is working much better for me."