Is it illegal to send prescription drugs through the mail?

Yes and no. It depends on the situation and reasoning. Mailing prescriptions are really only allowed by entities that are registered with the DEA, which is why you can order prescription drugs and send them back through the mail. It is also okay if your dad went on vacation and left his medication at home, you are permitted to send it to him. But any
other non special circumstance, if caught, can give you the same to similar consequences as selling them on the street.
 

Dialect is probably part of the answer