Purchase a small amount and see if it works. India in particular has a quite reputable pharmaceutical industry, but does not honour the extended U.S. patents on pharmaceuticals.
It depends upon what you can afford to loose, but if you need the medication to function, and you find a supplier which seems to be providing a reputable product for a fraction of the usurious prices you are being charged around the corner, and if you can purchase all sorts of other things via the internet or mail, with no regard for taking the food from your neighbors' children's mouths, why should this be any different.