Which of the following is cross-tolerant with heroin?

• A. Alcohol
• B. Morphine
• C. Marijuana
• D. Psilocybin

Answer: (B)

Cross-tolerance happens when tolerance to one drug makes you less sensitive to another drug because they act on the same system. Heroin is an opioid that mainly targets mu-opioid receptors in the brain. Morphine, another opioid, works on the same receptors. With repeated opioid use, the body adapts by diminishing receptor response, and that adaptation carries over to other opioids. So if you’ve become tolerant to heroin, you’ll typically also tolerate morphine better, meaning you’d need a higher dose to get the same effect.

The other options don’t share that direct receptor target. Alcohol acts on multiple systems (GABA, NMDA, etc.), while marijuana acts on cannabinoid receptors, and psilocybin on serotonin 2A receptors. Because these have different primary mechanisms, cross-tolerance with heroin isn’t as direct or reliable.