It is prohibited to mail prescription drugs, period. If caught doing so, your package will be seized and destroyed. Only drug manufacturers, pharmacies and other licensed dispensers may mail prescription drugs through the U.S. Postal Service. The only exception is the situation in which you're sending the drugs back to the manufacturer because of a recall or dispensing error.
TL;DR (Too Long; Didn't Read)
Except in very limited circumstances, it's illegal to mail prescription drugs anywhere, whether intrastate or across state lines.
Mailing Drugs Is Prohibited for Citizens
Only entities that are registered with the Drug Enforcement Administration are permitted to send drugs through the U.S. Postal Service. That's how you can order a prescription from a licensed pharmacist and have it arrive in the mail. Mailing prescription drugs is prohibited for citizens except in a few situations. It doesn't matter if you're trying to return medication that someone left at your house – it's against postal regulations to mail the drugs anywhere, whether within your own state or across state lines.
Product Recalls and Dispensing Errors
The only exception to the "no mailing" rule is when the drugs are recalled, or when the pharmacy sent you the wrong drug or dosage or made another kind of dispensing error. In that case, you can return the drugs, but only to the manufacturer or the manufacturer's agent. Speak to the manufacturer, doctor or pharmacist first, as they must send you a mailing container for the purpose of mailing back the drugs using the USPS "merchandise return" service.
DEA Mail-back Programs
Law enforcement and DEA-authorized mail-back collectors have the power to conduct mail-back programs. These programs allow the public to mail their surplus and unwanted pharmaceuticals to the mail-back collector for the purpose of safe disposal. It's legal to send your old prescription drugs through the mail if you are registered in a mail-back program. However, you must use the ready-made, pre-addressed packaging provided by the mail-back collector and you must follow its instructions for mailing back the package.
As the mailer, you are responsible for complying with USPS postal laws as well as federal, state and local laws regarding the mailing of prohibited substances. If you mail drugs and the USPS discover them in the mail stream, the drugs will be destroyed. In many states, giving your prescribed drug to someone else, even if you did not receive payment, is a felony punishable by fines and up to 15 years in prison. The person sharing your prescription drugs could also be charged with illegal possession. Receiving prescription drugs intended for another person is illegal in every case.