Which of the following hallucinogens was originally developed as a general anesthetic for surgery?

The hallucinogen that was originally developed as a general anesthetic for surgery is phencyclidine (PCP). PCP was synthesized in the 1920s and was primarily used as an anesthetic in medical settings due to its analgesic and sedative properties. Although it was initially intended for human use, its side effects, which included hallucinations and agitation, led to its discontinuation as an anesthetic in humans. Its impact as a hallucinogenic substance became evident over time, leading to its classification as a controlled substance.

While ketamine, another anesthetic, has hallucinogenic properties and is used in medical settings today, it was not the original hallucinogen to be developed for anesthesia. Marijuana and naltrexone are not classified as hallucinogens; marijuana is primarily known for its psychoactive effects due to THC, while naltrexone is an opioid antagonist used for treating substance use disorders. This context clarifies why phencyclidine is recognized as the correct answer related to its origins in surgical anesthesia.