Does Modafinil affect your weight? Will you gain or lose weight on Provigil?
We don't mean to cop out, but it is very difficult to say. Difficult to say for a lot of drugs. The body's weight control system, like the sleep cycle, is a complex mechanism, consisting of several (sometimes competing) forces. Some drugs make some people lose weight and other gain.
Modafinil is a stimulant, so you might think it would make you lose weight. It can substitute for amphetamine compounds in some clinical applications, and amphetamines often cause weight loss. However, modafinil is completely different and one of the salient points of this medicine is that it does not make people "wired". Just on that evidence, it seems less likely that modafinil would cause weight loss.
But is modafinil better than nothing when it comes to losing weight? Maybe. It effectively has antidepressant properties, so that could help some people eat less.
A 2003 clinical trial study concluded that "like amphetamine, modafinil
reduced the amount of food consumed and decreased energy intake, without
altering the proportion of macronutrients consumed" and further that
the amount of the medicine needed to affect appetite did not raise the heart
rate as amphetamines did.
A study at the Weight and Eating Disorders Program at the University of Pennsylvania said "Modafinil may be well suited for the treatment of obesity, although further studies with repeated dosing in overweight populations are warranted." Abstract here.
A study on Modafinil for depression found that the drug was associated with weight loss.
A Columbia University study found that for people doing shift work (which upsets the regular circadian cycle and eating patterns), Modafinil "produced clear reductions in food intake".