therapeutic window
Picture a comfortable temperature band: too cold and you shiver, too hot and you sweat, but in between you feel fine. The therapeutic window is that comfortable band for a drug's concentration in the blood — high enough to work, low enough not to poison.
Concretely, the window is the range of plasma drug concentrations bounded below by the minimum effective concentration and above by the concentration at which unacceptable toxicity begins. Dosing aims to keep the patient's concentration inside this band throughout treatment, which shapes how loading and maintenance doses and dosing intervals are calculated.
Whereas the therapeutic index is a single comparative ratio, the therapeutic window is the actual usable concentration range a clinician steers by, often read off from therapeutic drug monitoring. A narrow window — as for lithium or theophylline — means small fluctuations in absorption, clearance, or adherence can push a patient out of effectiveness or into toxicity, demanding careful titration.
Theophylline's therapeutic window is roughly 10–20 mg/L: below it the airways are not relaxed, above it nausea, arrhythmia, and seizures appear.
Stay inside the band, in both directions.
Index versus window: the therapeutic index is a unitless ratio used to compare drugs; the therapeutic window is a concrete concentration range (with units) used to dose one patient. Beginners often blur the two.