In animal studies, which drug is considered the most reinforcing, leading animals to work hardest to obtain it?

• A. Alcohol
• B. Cocaine
• C. LSD
• D. Heroin

Answer: (B)

The key idea is how strongly a drug motivates an animal to work for it, measured by self-administration and how high a breakpoint the animal will push under demanding schedules. Cocaine stands out because it rapidly floods the brain with dopamine by blocking the dopamine transporter, producing a fast, strong rewarding signal. This fast onset and potent dopaminergic effect make cocaine highly reinforcing, so animals will perform many lever presses and show high breakpoints to obtain it.

LSD generally shows weak reinforcing effects in animals and is not consistently self-administered, because its primary actions are on serotonin receptors rather than driving the mesolimbic dopamine reward system. Alcohol can be reinforcing, but its reinforcing strength is more variable and often lower in strict self-administration tests. Heroin is strongly reinforcing as an opioid drug, but in many standard paradigms cocaine yields higher motivation to obtain the drug, making cocaine the strongest reinforcer in those contexts.