It varies by state from totally illegal to totally legal and Virginia where Big Mama lives has legalized medicinal cannabis only for use in glaucoma and cancer patients.
You can do web searches to find out how the laws differ by state.

You are asking about "American" - federal law criminalizes pot broadly, and there is a conflict between state and federal laws in the states that allow some or all use of pot. It is probably way more than you ever want to know about the weird American federalism while not being an American.