Is it illegal to possess generic drugs?

First: NO, it is perfectly legal to import prescription drugs for which you have a prescription.

I think what you meant was: "Is it illegal to buy drugs from Another Country without prescription and import them?"

For America, typically yes, especially for stimulants and painkillers, which are always controlled substances because people always are trying to get their hands on them. See http://www.rxpop.com/legal.asp

I don't know how accurate the material is, but you can only import an FDA unapproved drug if you have a valid medical reason, and I guess the FDA expressly wants a "doctors note" to say it's OK, but not a prescription. But if you're importing a drug for an FDA approved reason, it says it's illegal.