From the following, which corresponds to quicker onset?

• A. Delayed onset: shift right
• B. Lower absorption: shift down
• C. Quicker onset: shift left
• D. Greater absorption: shift up

Answer: (C)

Onset is about how quickly the drug starts producing an effect after dosing. If you think of a graph of effect versus time, reaching the same level of effect sooner means the curve has moved to the left. So a quicker onset corresponds to a leftward shift, because the effect appears earlier. A rightward shift would mean the effect takes longer to appear (delayed onset). A downward shift implies a smaller effect at the same times, and an upward shift implies a greater effect at the same times, but neither necessarily means faster onset. Hence the quicker onset is represented by the leftward shift.