... (generic Adderal 20 mg tabs)? I have just switched from Nuvigil 250 mg (1 tab broken in 1/2) with 1/2 taken @ approx. 6 AM and the other 1/2 approx. 2-3 PM; to dextroamphetamine 20 mg-1 tab about 6am, the other tab @ mid-day (noon-1PM), and I'm starting to fall asleep at about 4:00 pm. What is a good rule of thumb for dosage conversion between the two drugs? I'm hoping my Dr will approve a third dose at about 4PM, because it only seems to last about four hours in my system, and I'm still working until 7 or 8! Someone HELP!!!