The GLP-1 "Dose Week" & PK Curve, Explained

The GLP-1 "dose week" is the predictable rise and fall of medication in your body between weekly injections, described by what pharmacologists call a PK curve. A PK curve, short for pharmacokinetic curve, is simply a graph of how much drug is in your bloodstream over time. Because semaglutide and tirzepatide have long half-lives, that curve climbs to a peak a few days after your shot, then drifts down toward a trough before the next one, and your appetite and side effects often loosely follow it.

Understanding this rhythm will not change your dose, and nothing here is dosing advice. What it does is help you make sense of why some days feel different from others, so you can plan your eating and training around your own pattern.

What the "dose week" actually means

A weekly GLP-1 injection is not a switch that stays evenly on for seven days. It is a single input that the body absorbs, distributes, and slowly clears. The amount in your bloodstream is highest a few days after the shot and lowest right before the next one. That repeating arc, from peak to trough and back, is the dose week.

Two pharmacokinetic terms make this concrete. Half-life is the time it takes for the amount of drug in your body to fall by half. Tmax is the time after injection when blood concentration reaches its peak. Together they describe the shape of the curve: how fast it rises and how slowly it falls.

The reason the curve is so smooth for these medications is their unusually long half-lives, which is exactly why they are dosed once a week instead of daily. A drug that cleared quickly would force frequent dosing and sharp swings; a long half-life flattens the ride.

Semaglutide vs tirzepatide: half-life and the shape of the week

Semaglutide (Ozempic/Wegovy) and tirzepatide (Mounjaro/Zepbound) are both once-weekly, but their pharmacokinetics differ slightly, which subtly changes the shape of the dose week. The table below summarizes the published figures. All values are approximate population numbers from pharmacokinetic data, and your individual experience may not match them.

Property	Semaglutide (Ozempic/Wegovy)	Tirzepatide (Mounjaro/Zepbound)
Half-life	~7 days	~5 days
Time to peak (Tmax)	~1 to 3 days after injection	~1 to 3 days after injection
Time to steady state	~4 to 5 weeks of consistent dosing	~4 weeks of consistent dosing
Roughly detectable after last dose	~5 weeks	~4 to 5 weeks
Typical appetite pattern reported	Often strongest early in the week, easing toward the next dose	Often strongest early in the week, easing toward the next dose

Source: PNAS 2024 (doi:10.1073/pnas.2415815121) and product pharmacokinetic references.

The headline difference is the half-life. Semaglutide's roughly 7-day half-life is close to its dosing interval, so the curve is relatively flat across the week. Tirzepatide's roughly 5-day half-life means a somewhat steeper decline late in the week for some people. Neither is "better"; they are just shaped differently, and that shape can influence how the back half of your week feels.

What "steady state" changes

For the first several weeks on a stable dose, each weekly injection adds onto residual drug from the previous one, so average levels climb week over week until they plateau. That plateau is steady state, generally reached around 4 to 5 weeks of consistent dosing. Tirzepatide's steady-state concentration is roughly 2.1 times its first-dose concentration, according to pharmacokinetic data, which is a useful reminder that the early weeks are not representative of where you will settle.

The practical point: if your dose week feels different a month in than it did at the start, steady state is a likely reason, not anything you did wrong. As always, anything that concerns you about how a dose is affecting you belongs in a conversation with your prescriber.

Why your curve is not the textbook curve

Published half-lives and Tmax values are population averages, and your personal curve can sit some distance from them. Body size, kidney and liver function, injection site and technique, how long you have been on the medication, and simple biological variation all shift the real shape of your week. Two people on the same dose of the same drug can describe genuinely different experiences, and both can be normal.

That is why the most useful curve is not the one in a pharmacology paper; it is the one you build from your own observations. The textbook version tells you roughly what to expect, a peak a few days out and a softer back half. Your own logged pattern tells you what actually happens, which is the version you can plan around. Treat the published numbers as a starting hypothesis and let your own data refine it.

Peak and trough: why appetite tracks across the week

Peak is the high point of the curve, a few days after your shot. Trough is the low point, just before the next dose. Many people describe appetite suppression as strongest near the peak and easing toward the trough, with "food noise" and hunger creeping back in the final day or two of the week.

This is a commonly reported pattern, not a guarantee, and the science here deserves a hedge. Appetite is influenced by far more than blood drug levels, including sleep, stress, activity, and the meal in front of you. The curve is a reasonable mental model for why the end of the week can feel harder, but it is not a precise predictor of any given day.

Side effects can loosely follow the curve too. Some people notice nausea or GI symptoms cluster in the days right after a dose, when levels are climbing toward peak. Tracking how your symptoms map onto your dose timing is the whole premise of keeping a GLP-1 side effects log, and it is the only way to learn your personal pattern rather than the population average.

Using the curve to protect muscle

Here is where the dose week stops being trivia and starts being useful. The days your appetite returns are not a problem to endure; they are an opportunity. When eating feels easier, that is your window to hit protein, the single most important nutritional lever for keeping muscle while you lose weight.

The logic is simple. Appetite suppression on a GLP-1 cuts calorie intake, and protein is usually the macro that slips, which is a core driver of why GLP-1 weight loss can include a meaningful share of lean mass. A 2025 study found fewer than half of GLP-1 users hit the minimum recommended protein target of 1.2 g/kg per day (Johnson et al., 2025, Nutrition). Treating your easier-appetite days as protein days is one practical way to close that gap. Our guide on how to hit your protein goal with no appetite covers the tactics, and the deeper protein on a GLP-1 piece covers the targets.

Training can flex around the curve too. Some people prefer to schedule harder resistance sessions for the part of the week they feel strongest and best fueled. There is no universal "right" day; the point is to notice your pattern and work with it instead of fighting it. None of this involves changing your dose or timing, which stays exactly as your prescriber set it.

Why the trough days are the sneaky ones

The risk in the dose week is not the peak; it is the back half. On low-appetite early-week days, eating little feels fine, and on returning-appetite late-week days, it is easy to swing the other way. Either way, the days your protein quietly falls short are the days your muscle-preservation effort slips, and they are invisible unless you are looking.

This is precisely what Myo is built to surface. Its medication-level PK curve and personal "Your Body's Curve" map your dose week, and the GLP-1 week phases, Shot Day, Peak, Stable, Fade, and Prep, label where you are in that arc. Crucially, Myo lets you see your protein intake mapped across those phases, so you can catch the low-appetite days where your muscle-protecting nutrition is slipping before they add up. Pairing the curve with consistent injection tracking gives the model a clean timeline to work from.

The curve is one of the few genuinely predictable things about a GLP-1. Learn its shape, respect the trough, and use the easier days, and you turn an abstract pharmacokinetic graph into a weekly plan that protects the muscle you are working to keep. For how that fits the bigger muscle-loss picture, start with the complete guide to GLP-1 and muscle loss.

References

• PNAS (2024). Pharmacokinetics of semaglutide and GLP-1 receptor agonists. doi:10.1073/pnas.2415815121.
• Product pharmacokinetic data for once-weekly semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro, Zepbound), summarized in trimrx semaglutide half-life and pharmacy tirzepatide pharmacokinetic references.
• Johnson et al. (2025). Protein intake in GLP-1 users. Nutrition. PMC12419545.
• Manufacturer prescribing information for each respective GLP-1 product (the authority for dosing, timing, and missed-dose handling).