Provigil (modafinil) and Nuvigil (armodafinil) are stimulant-like drugs used to treat excessive sleepiness. There are generics available for Provigil but not for Nuvigil. They are manufactured by the same company (Cephalon) and have the same effects. Armodafinil (Nuvigil) is an enantiopure drug; it is the R-enantiomer of modafinil (Provigil).
Both Provigil and Nuvigil increase the release of monamines, specifically catecholamines norepinephrine and dopamine, from synaptic terminals. They also elevate hypothalamic histamine levels.
In terms of chemical composition, armodafinil is an enantiomer of modafinil. It is possible that enantiomers of drugs bind differently to receptors in the body and therefore have different effects. For example, one enantiomer of Naproxen is used to treat liver pain but another causes liver poisoning with no analgesic effects. However, in the case of modafinil (Provigil) and armodafinil (Nuvigil), both drugs have similar effects on the body.
Since the two drugs are not chemically identical, the patent application for Nuvigil was filed and approved separately. The patent for Provigil has expired and generics are available.
Both Provigil and Nuvigil are used to treat narcolepsy, obstructive sleep apnea and shift work disorder.
In addition, Provigil may be prescribed to treat ADHD, mood disorders, depersonalization disorder, cognitive enhancement, fatigue, cocaine addiction, post-chemotherapy cognitive impairment, weight loss, as a doping agent and in military and law enforcement.
Nuvigil was being considered as treatment for jet lag, schizophrenia and depression but failed to meet FDA approval for all of these uses.
In the case of OSA and narcolepsy, Provigil should be taken as 200mg single dose in the morning and to treat SWD, 200mg tablet should be taken 1 hour prior to shift.
Nuvigil is prescribed as 150mg/250mg single dose tablet in the morning for patients with OSA and narcolepsy. To treat SWD, it is taken as 150mg tablet prior to work shift.
Studies have shown that wakefulness of patients treated with Provigil and Nuvigil improves significantly in the case of narcolepsy, shift work disorder (SWD) and obstructive sleep apnea (OSA) when compared to a placebo.  
When compared to each other, however, no differences have been found in the efficacy of provigil and nuvigil.
edit Side Effects
The common side effects of Provigil include, back pain, headache, nausea, nervousness, stuffy nose, diarrhea, anxiety, dizziness, upset stomach and trouble sleeping. The more serious effects include serious rash, serious allergic reaction involving the liver or blood cells, hives, mouth sores, blistering or peeling skin, swelling of the face, lips, eyes, tongue, legs or throat, trouble swallowing or breathing, fever, shortness of breath, yellowing of the skin or whites of eyes, dark urine.
The common side effects of Nuvigil are headache, nausea, dizziness, trouble sleeping. The serious side effects are similar to the use of Provigil.
Provigil and Nuvigil are not approved for pediatric use. In clinical trials of modfanil, serious rash requiring hospitalization and discontinuation of treatment has been reported. The rashes included one case of possible Stevens-Johnsons-Syndrome (SJS) and a case of multi-organ hypersensitivity reaction.
No serious withdrawal symptoms were noticed in patients. The condition of excessive sleepiness returned when intake was stopped.
A study of the abuse potential of Provigil has demonstrated that modafinil produced psychoactive and euphoric effects and feelings consistent with other scheduled CNS stimulants like methylphenidate. The abuse potential of Nuvigil has not been specifically studied and is likely to be similar to that of Provigil.
This video explains the use and abuse of modafinil (Provigil).