Half-Life

Half-Life

In pharmacology, a drug’s half-life (t1/2) is the time it takes for its blood concentration to fall to 50% of its peak. It is a basic pharmacokinetic measure that decides how often a drug must be taken: drugs with short half-lives need frequent doses, while drugs with long half-lives can be taken less often.

Half-life depends on two main factors: clearance (how fast the body removes the drug) and volume of distribution (how widely the drug spreads into body tissues). For peptide drugs like retatrutide, chemists use techniques such as attaching fatty acid chains and changing amino acids to make the half-life far longer than that of the natural hormones. The body’s own GLP-1 has a half-life of about two minutes; retatrutide’s engineered structure gives it a half-life of roughly six days, which allows a once-weekly injection under the skin.

Half-life also explains steady-state levels. For any drug taken at regular intervals, it takes about four to five half-lives to reach a steady concentration in the blood. For retatrutide, that means about four to five weeks of weekly doses before drug levels settle. This affects when treatment effects should be expected and how dose-escalation schedules are designed in clinical trials.