Today is my first day switching from Adderall (20mg twice a day) to Nuvigil (150mg) once a day. I have narcolepsy. The Adderall was working great for me except that I kept losing weight and didn't have much appetite.

It has been four hours since my first dose of Nuvigil at 150mg. I feel terrible. I have a major headache, nausea and feel like I'm going to fall over from exhaustion. It was hard for me to walk up the stairs and I am very fit.

What would be causing this?