Half-life

Half-life is the time it takes for half of a drug to clear your system. It is the number that explains why one GLP-1 is injected weekly, another daily, and a third swallowed every morning — and why stopping one takes a month while stopping another takes a few days.

The spread across this category is enormous. Your own GLP-1 lasts 1.5 to 2 minutes before enzymes destroy it. Liraglutide, engineered to resist those enzymes, lasts about 13 hours — hence once-daily Saxenda. Tirzepatide runs about five days. Semaglutide runs about a week, and its labels state that after a final 2.4 mg injection the drug remains in circulation for roughly five to seven weeks. Orforglipron, the small-molecule tablet, clears in about 29 to 49 hours.

A rough rule helps here: a drug is essentially gone after four to five half-lives. That is why the Wegovy label instructs patients to stop at least two months before a planned pregnancy, citing the long half-life of semaglutide explicitly. It is also why side effects do not end on your last injection day, and why a “wearing off” feeling late in the week is a real phenomenon on some drugs and not others.

The confusion worth correcting is equating half-life with how long the effect lasts. Appetite suppression and slowed stomach emptying do not track the blood level one-for-one — the Zepbound label notes that the gastric-emptying delay is largest after the first dose and diminishes over time, even though the half-life does not change. Steady state, the point where each dose replaces what cleared, takes about four weeks on the weekly drugs.

Half-life is the right thing to raise when planning surgery, pregnancy, a switch between drugs, or a deliberate stop. Ask your clinician how long before each of those you should stop. Our washout calculator and half-life explainer do the timing.