Half-life

Half-life is a pharmacology term for how long it takes your body to clear half of a dose of a drug. It’s the single number that explains the entire weekly rhythm of GLP-1 treatment.

Why weekly dosing works

Semaglutide has a half-life of about 7 days; tirzepatide about 5. That’s extraordinarily long for a drug — engineered deliberately, since natural GLP-1 lasts only minutes. A week-long half-life means the medication level in your body stays relatively steady between weekly shots, rather than spiking and crashing. That steadiness is why shot day is weekly instead of daily.

Why missed doses have windows

The long half-life is also why a late dose usually isn’t a crisis. Miss your shot by a day or two and enough drug remains that levels stay in a reasonable range — which is exactly why each medication’s label defines a missed-dose catch-up window (Wegovy: within 48 hours of the next dose; Ozempic: 5 days; tirzepatide: 4 days).

Why steady state takes weeks

There’s a flip side. It takes roughly 4–5 half-lives to reach a stable level after starting or changing a dose — for semaglutide, about 4–5 weeks. This is part of why titration steps are spaced a month apart, why the full effect of a dose increase isn’t immediate, and why restarting after a long gap can feel like starting over: the level has to rebuild.