Alcohol Drugs and Society Practice Test 2026 – Complete Exam Prep

Cross-tolerance happens when two drugs share the same mechanism of action, so changes in the body from using one drug reduce the response to the other. Heroin and morphine are both opioids that activate the mu-opioid receptors in the brain. Since heroin is rapidly converted to morphine, the brain’s adaptations that reduce sensitivity to heroin also blunt the effects of morphine. That shared receptor system means tolerance to one drug translates into tolerance to the other, which is exactly what cross-tolerance describes.

The other substances act on different systems—alcohol mainly affects GABA and NMDA receptors, marijuana targets cannabinoid receptors, and psilocybin influences serotonin receptors. Because they engage different targets, the tolerance developed to heroin doesn’t automatically produce tolerance to those drugs in the same way.