GLP-1 medication levels explained: why your week has a shape

If you’ve been on a weekly GLP-1 for more than a month, you’ve probably noticed it: the week has a shape. Maybe appetite creeps back every weekend. Maybe day 2 is reliably your queasiest. That shape isn’t your imagination — it’s pharmacokinetics.

The curve behind every dose

When you inject a weekly GLP-1, the medication doesn’t appear in your blood instantly. It absorbs from the injection site over hours to days, reaches a peak (typically 1–3 days post-injection), then declines as your body slowly eliminates it.

Two numbers define this curve:

• Time to peak: how long absorption takes to overtake elimination.
• Half-life: how long it takes your body to clear half the medication — about 7 days for semaglutide and roughly 5 days for tirzepatide.

Because the half-life is about as long as the dosing interval, each new dose lands on top of what’s left from previous ones. Over the first 4–5 weeks at a steady dose, levels stack up to a plateau — which is why effects (and side effects) often build over the first month at each dose step.

What the shape explains

• Post-shot side-effect windows. Nausea and fatigue cluster near the peak, in the first days after injecting. Trial safety data shows the same pattern.
• Pre-shot hunger. By days 6–7 you’re at the weekly trough — for many people the hungriest stretch. It’s not failure; it’s the curve.
• The titration bump. Each dose increase restarts the build-up at a higher plateau, which is why step-up weeks can feel like week one again.

What a level estimate is — and isn’t

Apps like ShotLock model your estimated level by applying published absorption and half-life values to your actual logged shots — including the late ones. That makes the weekly rhythm visible and explains a lot of “why do I feel off today?” moments.

But be clear about what it is: a model of the typical curve, not a blood measurement. Individual absorption varies, and no app can capture your personal pharmacology. Use level curves to understand patterns and time your routine — never to make dosing decisions, which belong to you and your prescriber.

Seeing your own curve

The interesting part isn’t the textbook curve — it’s yours. Log shots consistently for a few weeks and patterns emerge: how your symptom logs line up with your modeled peak, how a late shot deepened the trough, how the plateau settled after your last step-up. ShotLock’s med-levels view draws exactly this, phase by phase, from your own history — with every parameter cited in its Research Library.

Estimates are informational, not medical advice. Talk to your prescriber about anything dosing-related.