How is medication a type of antecedent control?
It is taken to control an "event" before it occurs.
"Antecedent" means to come before (like "ante" in poker). Some medications are only taken after symptoms are observed. In psychiatry (psychologists may not prescribe medications, only psychiatrists) we often prescribe medications to prevent the onset of symptoms.
Examples would be anti-depressants or bipolar disorder medications.