If a drug's side effects increase with dosage, what does that suggest about its mechanism of action?

The correct answer suggests that if a drug's side effects increase with dosage, it is likely antagonizing dopamine receptors. When a medication functions as an antagonist at dopamine receptors, this interaction often leads to various side effects, especially as the dose increases. Dopamine antagonism is associated with adverse effects that can emerge or worsen with higher doses, including extrapyramidal symptoms, sedation, or other neurological effects, commonly seen with antipsychotic medications.

In contrast, enhancing dopamine activity, inhibiting serotonin activity, or activating serotonin receptors would not typically cause a proportional increase in side effects directly tied to the dosage; rather, these mechanisms may have a more consistent side effect profile that doesn't escalate dramatically with increased doses. This characteristic of antagonistic drugs supports the idea that the side effects reflect the intensity of the pharmacological action, which can lead to an increased likelihood of adverse events at higher exposures.