Which of the following medications does not require a prescription?

Diphenhydramine is an over-the-counter medication that is commonly used as an antihistamine to alleviate allergy symptoms, such as sneezing, runny nose, and itchy eyes. It is also used as a sleep aid due to its sedative properties. Because it is available without a prescription, it can be purchased directly by consumers at pharmacies and supermarkets.

The other medications listed require a prescription due to their potential for misuse or the need for medical supervision. Pseudoephedrine, while available without a prescription in some contexts, is regulated due to its use in the illicit production of methamphetamine, often requiring identification for purchase. Amoxicillin is an antibiotic that requires a prescription to ensure appropriate use for bacterial infections and to combat antibiotic resistance. Lisinopril is an antihypertensive medication that also necessitates a prescription because it must be monitored by a healthcare professional due to possible side effects and interactions with other medications. Thus, diphenhydramine stands out as the only medication in this list that does not require a prescription.