A patent medicine, sometimes called a proprietary medicine, is an over-the-counter (nonprescription) medicine or medicinal preparation that is typically protected and advertised by a trademark and trade name (and sometimes a patent) and claimed to be effective against minor disorders and symptoms. Its contents are typically incompletely disclosed. Antiseptics, analgesics, some sedatives, laxatives, and antacids, cold and cough medicines, and various skin preparations are included in the group. The safety and effectiveness of patent medicines and their sale is controlled and regulated by the Food and Drug Administration in the United States and corresponding authorities in other countries.

The term is sometimes still used to describe quack remedies of unproven effectiveness and questionable safety sold especially by peddlers in past centuries, who often also called them elixirs, tonics, or liniments. Current examples of quack remedies are sometimes called nostrums or panaceas, but easier to understand terms like scam, cure-all, or pseudoscience are more common.

Patent medicines were one of the first major product categories that the advertising industry promoted; patent medicine promoters pioneered many advertising and sales techniques that were later used for other products. Patent medicine advertising often marketed products as being medical panaceas (or at least a treatment for many diseases) and emphasized exotic ingredients and endorsements from purported experts or celebrities, which may or may not have been true. Patent medicine sales were increasingly constricted in the United States in the early 20th century as the Food and Drug Administration and Federal Trade Commission added ever-increasing regulations to prevent fraud, unintentional poisoning and deceptive advertising. Sellers of liniments, claimed to contain snake oil and falsely promoted as a cure-all, made the snake oil salesman a lasting symbol for a charlatan.