Intensity of a drug's response is proportional to which variable?

• A. AUC
• B. Tmax
• C. Peak absorption rate
• D. Plasma concentration

Answer: (D)

The amount of drug available at the site of action determines how strong the response will be at any moment. Plasma concentration directly reflects how much drug is present in the body to reach receptors and trigger effects, so higher plasma levels generally produce a greater response (up to saturation). Total exposure over time, shown by AUC, relates to overall dose and duration rather than the instant intensity of the effect. Tmax indicates when the peak concentration occurs, not how big the effect is. Peak absorption rate describes how quickly the drug enters the bloodstream, which affects timing more than magnitude. Therefore, plasma concentration is the best measure of the immediate intensity of the drug’s response.