Any drug prescribed by a Physician is legal. Any drug that is not given by a Physician or bought on the street are illegal. Example: Oxycontin given to a person by his/her Doctor is legal, Oxycontin obtained without a prescription is illegal.
Give an example of a drug that is both illegal and legal?
Marijuana ! It's illegal in some places and legal in others !
If you use a drug for what it is intended then it is legal. If you use it for other than it was intended it is illegal. So if you smoke marijuana for glaucoma or another medical problem (providing you've been approved) then it's legal, if its for recreational use then it's illegal. The same can be said for more commonly prescribed drugs like morphine or something as simple as gravol. If you take as prescribed it's legal or for fun illegal.