dosing interval
The dosing interval is simply the gap between one dose and the next — every 8 hours, once a day, weekly. It is the rhythm of a regimen, written in pharmacokinetic shorthand as tau. Together with the size of each dose, it determines both the average drug level and how much the level rises and falls between doses.
Choosing the interval is a balancing act usually anchored to the half-life. Dosing roughly once per half-life keeps peak-to-trough swings moderate; much longer intervals let the level dip into the sub-therapeutic range before the next dose; much shorter intervals flatten the swings but burden the patient. The total daily amount stays the same as long as dose-per-interval scales with interval length.
Drugs with a wide therapeutic window tolerate long intervals and big swings, which is why convenient once-daily dosing is feasible. Drugs with a narrow window often need either frequent doses or a slow-release formulation to keep peaks below the toxic ceiling and troughs above the effective floor. Real-world intervals also bow to practicality, since regimens that are simpler to follow improve adherence.
Short-half-life drugs like immediate-release morphine need dosing every 4 hours, whereas long-acting fluoxetine can be taken once daily — the interval tracks the half-life.
The interval typically follows the half-life.
Halving the interval and the dose together keeps the average level the same but halves the peak-to-trough swing — handy for narrow-window drugs.