Which among the following is not an Analgesic?

The correct option is Ranitidine.

To understand why this is the correct choice, let us look at the definition of analgesics and examine each of the given options:
Analgesics are drugs that are used to relieve pain without causing loss of consciousness, mental confusion, incoordination, or paralysis. They are commonly known as pain relievers.

Now, let us analyze each drug listed in the options:
1. Morphine: It is a potent opioid analgesic. It acts directly on the central nervous system (CNS) to decrease the perception of pain.
2. Heroin: Also known as diacetylmorphine, it is a semi-synthetic opioid drug derived from morphine. It is a highly addictive analgesic.
3. Codeine: It is another naturally occurring opioid (alkaloid found in the opium poppy) used as an analgesic to treat mild to moderate pain.
4. Ranitidine: Unlike the others, Ranitidine is not a pain reliever. It belongs to a class of drugs known as H₂-histamine receptor antagonists (H₂ blockers). It works by reducing the amount of acid produced in the stomach. Therefore, it is used to treat indigestion, heartburn, and gastroesophageal reflux disease (GERD), making it an antacid/anti-ulcer drug rather than an analgesic.

Hence, among the given options, Ranitidine is not an analgesic.