The answer depends on where you plan on selling, but in general you will need a license to start your business, then a license to sell pharmaceuticals.
If you plan on selling in the US, for example, first you will have to get a pharmacy license in your home state so you can open and maintain a pharmacy. Then, you will need to get a license to sell in every other state in which you plan to sell. You will also need to make sure that you comply with all regulations for every state in which you do business.
Basically, it will take a lot of work to make sure that you comply with all regulations because pharmaceuticals are highly regulated. I suggest that you speak with an attorney before you start your online medical store. If you have more questions and are looking for an attorney, check out. Our attorneys have experience with business formation and the initial consultation is free, Good luck!