therapeutic window

The therapeutic window is the safe lane between 'too little' and 'too much'. Below it, the drug does not work; above it, it starts to harm. Picture seasoning a soup: too little salt and it is bland, too much and it is inedible, and there is a comfortable range in between. The therapeutic window is that range for a drug's dose or blood concentration.

More formally, it is the span of doses (or plasma concentrations) that lies above the level needed for a useful effect and below the level where unacceptable toxicity appears. A wide window is forgiving — small dosing errors do not matter much. A narrow window is dangerous — the same milligram that helps one day can harm the next, so such drugs often need blood-level monitoring and careful individualized dosing.

The window is shaped by both the desired effect's dose–response curve and the toxicity's dose–response curve; the gap between them is what you are trying to exploit. It is not a fixed physical constant: it can shrink in patients with impaired metabolism, in the very old or very young, or when another drug competes for the same metabolizing enzymes. Medicinal chemists work hard early on to design molecules whose useful and harmful effects are pried as far apart as possible.