Semaglutide & tirzepatide half life

Verified May 10, 2026: When the FDA labels a GLP-1 receptor agonist as "weekly," it means the terminal elimination half-life is long enough that a once-weekly dose maintains a therapeutic blood level. The drug is not gone after seven days. Reading the half-life curve directly, instead of treating doses as discrete weekly events, is what separates titration tracking from injection logging.

What "half-life" means

The plasma half-life of a drug is the time it takes for the blood concentration to fall by 50%. After one half-life, you have 50%. After two, 25%. After five half-lives, you have ~3% — clinically negligible for most drugs.

This is the terminal half-life: the slow elimination phase, not the absorption phase. For long-acting GLP-1s, absorption is essentially complete within 24-48 hours, then the slow decay begins.

The actual half-lives by compound

Liraglutide is the outlier: a daily injection, because its half-life is too short to support weekly dosing. Everything in the long-acting weekly family sits in the 5-7 day range.

Why steady-state takes 4-5 weeks

Steady-state is the point where the amount of drug entering your system per dose equals the amount being eliminated. For weekly GLP-1s, steady-state is reached after ~4-5 half-lives — which is 4-5 weeks from your first dose at any new step.

This is why the FDA-approved ramps move every 4 weeks: it gives the previous step time to reach steady-state before the next bump. Moving faster stacks peaks. See the semaglutide dose schedule and tirzepatide dose schedule for the full ramp logic.

What it looks like when you miss a dose

For semaglutide (7-day half-life): missing one dose drops your blood concentration to about 50% of steady-state by day 14, and 25% by day 21. The drug is still doing something at day 14, so you don't restart the ramp — you just take your next scheduled dose at the original strength.

For tirzepatide (5-day half-life): the same miss drops you faster. You'll feel the appetite return on day 9-10. Resume at the same dose unless you've missed two consecutive weeks, in which case the FDA label suggests stepping back one rung.

What the chart tells you that the calendar can't

A weekly calendar entry tells you when you took your dose. It doesn't tell you what your blood concentration was when you logged a side effect on day 3. That matters: nausea on day 3 of a fresh dose differs from nausea on day 6 of a missed dose.

The plasma-decay curve, plotted from the actual pharmacokinetics, lets you anchor every side effect to where you were on the curve. After two months of data, the patterns become obvious — your tolerable window, your peak-effect days, the third day of every dose where dinner is hard to finish.

How Titrate visualizes this

Every compound in Titrate's library ships with its published half-life. The 14-day chart on the Insights screen plots your actual dose log against the predicted curve. Multi-compound stacks overlay so you can read interaction windows. Side-effect markers anchor to the curve at the time you logged them.

The chart tells you what the calendar can't: where the molecule actually is, today, in your blood.

Frequently asked questions

What is steady-state for a GLP-1?

The point where drug entering your system per dose equals drug being eliminated. For weekly GLP-1s with a 5-7 day half-life, steady-state is reached after 4-5 weekly doses at the same step.

Why does the FDA ramp every 4 weeks?

To let the previous step reach steady-state before adding more. Moving faster stacks peaks and produces unmanageable side effects.

What happens if I miss a weekly dose?

For semaglutide: blood concentration drops to ~50% by day 14. Take the next scheduled dose at the regular strength; do not restart the ramp. For tirzepatide: appetite return is more noticeable. If two consecutive weeks are missed, FDA label suggests stepping back one rung.

Why is liraglutide a daily injection?

Its terminal half-life is only ~13 hours — too short to maintain a therapeutic blood level on weekly dosing. Daily dosing is required to keep the drug at steady-state concentration.