Cocaine is known to reduce the effectiveness of beta blockers because it acts as a stimulant that increases heart rate and blood pressure through its adrenergic effects. Beta blockers work by blocking the effects of adrenaline on the heart, which leads to a decreased heart rate and lower blood pressure. When cocaine is present in the system, it counteracts the actions of beta blockers, creating opposing cardiovascular effects.
In contrast, substances like heroin, alcohol, and nicotine have different action mechanisms that do not directly undermine the therapeutic effects of beta blockers in the same way that cocaine does. Heroin is an opioid with sedative effects, alcohol can alter cardiovascular responses but typically doesn't have a direct antagonistic effect on beta blockers, and nicotine primarily acts as a stimulant but does not possess the same capacity to negate the effects of beta blockers as cocaine does. Therefore, cocaine's unique properties in this context make it the substance that reduces the effectiveness of beta blockers due to its opposing cardiovascular effects.