How to Track GLP-1 Medication Levels Between Injections
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GLP-1 medications like semaglutide and tirzepatide have a known half-life, which means you can estimate how much is active in your body at any point between weekly injections. The level peaks in the day or two after your shot, then declines in a predictable curve until your next dose. Plotting that curve based on your injection day and dose gives you a running picture of your medication level across the week.
Why medication levels change throughout the week
Weekly GLP-1 medications do not deliver a flat, constant dose. After you inject, the drug is absorbed and reaches a peak concentration, then your body gradually clears it over the following days. Because the half-life of semaglutide is roughly seven days, you still have a meaningful amount in your system by the time your next injection is due, but the level is noticeably lower than it was a day or two after the shot.
This explains why some people feel stronger appetite suppression mid-week and notice it fading toward injection day. Knowing where you are on that curve helps you understand your own experience and make sense of hunger or energy changes that might otherwise seem random.
How to calculate your estimated level manually
The core concept is pharmacokinetic decay based on half-life. For semaglutide, the half-life is approximately 165 to 184 hours (about seven days). That means if you inject on Monday, you still have roughly half the peak concentration the following Monday before your next shot. Each day between those points, the concentration decreases by a predictable fraction.
To sketch the curve yourself: note your injection day, treat the peak as occurring roughly one to three days after injection, then apply the decay formula. The amount remaining at any time T after your peak equals the peak amount multiplied by 0.5 raised to the power of (T divided by the half-life in the same time units). This gives you a number relative to your dose that you can plot by hand on graph paper or in a spreadsheet.
Building a simple weekly level chart
A spreadsheet is the most practical way to visualize this without special tools. Set up a column for each day of your injection cycle (day 1 through day 7), enter the half-life value for your specific medication, and use the decay formula to fill in a relative concentration for each day. Chart that column as a line graph and you will see the curve that shows your estimated level rising after injection and tapering toward the end of the week.
If you are on a medication like tirzepatide, the half-life is roughly five days, so the decay is somewhat steeper across the week. Adjust the half-life input in your formula accordingly. The visual curve is what matters most: it lets you identify which days you are likely near peak levels and which days you are approaching the trough before your next dose.
Accounting for steady-state and dose changes
After several weeks on the same dose, your body reaches a steady state where the residual medication from previous injections adds to each new dose. This means the effective weekly curve shifts upward compared to your very first injection. The shape of the curve stays similar, but the baseline never drops to zero because prior doses have not fully cleared.
When you increase your dose, the steady-state level resets upward over the following four to five weeks. If you are tracking your levels and you notice the curve seems to feel different after a dose change, that gradual build to a new steady state is the likely reason. Tracking week over week during a titration period can help you see this accumulation visually.
Practical uses for knowing your level curve
Understanding where you are on the curve helps you plan. If you know your medication is at its lowest on injection eve, you can expect more hunger on that day and prepare accordingly rather than interpreting it as a failure of the medication or your habits. Some people shift their injection day to better align peak levels with days that are socially or professionally demanding.
The curve also provides context when you talk to your prescriber. Instead of saying 'I feel hungry sometimes,' you can describe specifically that you notice increased appetite on days five through seven of your cycle, which is a much more useful data point for evaluating whether your current dose is appropriate.
What people run into
it shows a chart with estimated medication levels over time between injections
Helps me see how much of the medication is in my system
Questions people ask
What is the half-life of semaglutide and how does it affect weekly levels?
Semaglutide has a half-life of about seven days (roughly 165 to 184 hours). This means that by the time your next weekly injection is due, you have about half the peak concentration remaining. The level is not zero, so there is always some medication active, but it is at its lowest point just before each new shot.
Is there a way to see a visual curve of my GLP-1 levels without doing the math myself?
Yes. A simple spreadsheet with a decay formula and a line chart accomplishes this without manual calculation. Enter your injection day, your medication's half-life, and apply the exponential decay formula across each day of the cycle, then chart it. The result is a visual curve you can reference throughout the week.
Why do I feel hungrier right before my next injection?
Your medication level is at its lowest point just before your next weekly dose. Because GLP-1 agonists reduce appetite in part by slowing gastric emptying and acting on hunger signaling, lower levels late in the week mean less of that effect, so increased hunger in the day or two before your injection is a normal and expected result of the medication cycle.
Does the level curve change when I increase my dose?
The shape of the curve stays roughly the same, but the overall level shifts upward. After a dose increase, it takes about four to five weeks to reach a new steady state as residual medication from each injection accumulates. During that period, your effective weekly level gradually rises before stabilizing.
Does tirzepatide have a different curve than semaglutide?
Yes. Tirzepatide has a shorter half-life of approximately five days compared to semaglutide's seven days. This means the level drops more steeply across the week, so the trough before your next injection is lower relative to the peak. The same decay formula applies, but you use five days as the half-life value instead of seven.
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