GLP-1 Half-Life Calculator

What is GLP-1 Half-Life?

A drug's half-life is the time it takes for half of it to leave your body. Semaglutide has a half-life of about 7 days and tirzepatide about 5 days. That is why both are injected once weekly: enough medication remains between doses to keep working, so levels never crash to zero.

The number matters less than what it implies. A long half-life means each injection overlaps with the last, so levels build for the first month of any dose before settling into a stable weekly rhythm. It also means the drug fades gradually rather than switching off, which is exactly why your appetite feels different on day 2 than on day 6.

How Long Does Semaglutide Stay In Your Body?

Semaglutide takes roughly 5 to 7 weeks to clear almost completely after your final injection, because a drug is considered essentially gone after about five half-lives and semaglutide's half-life is about 7 days. Meaningful appetite suppression usually fades within 2 to 4 weeks, well before the drug is fully gone.

Within a normal week of treatment the picture is different. Seven days after an injection, roughly half of that dose is still in you, which is exactly why the once-weekly schedule works. This applies equally to Ozempic and Wegovy, because both are semaglutide.

How Long Does Tirzepatide Stay Active?

Tirzepatide has a half-life of about 5 days, so it clears almost completely in roughly 4 weeks after your last dose. Between weekly injections, about 38 percent of each dose is still present when the next one is due, compared with 50 percent for semaglutide. Levels fall a little faster through the week.

In practice this is why some people on Mounjaro or Zepbound notice hunger returning slightly earlier in the week than friends on semaglutide. It is a property of the molecule, not a sign that your dose has stopped working.

Why Appetite Changes During The Week

Your appetite tracks your drug level, and your drug level falls every day between injections. Levels peak in the first 24 to 48 hours, stay high through mid-week, then decline toward their lowest point on days 6 and 7. Hunger returning late in the week is the curve working normally, not the medication failing.

This is the single most common worry on GLP-1 forums, and it has a boring answer: you are feeling the shape of the curve. Someone who injects on Sunday and feels ravenous on Saturday is not developing tolerance. They are at the bottom of their weekly trough, hours away from resetting it.

Two things make that trough feel worse than it needs to. Under-eating protein earlier in the week leaves you genuinely hungrier, and a dose that has not yet reached steady state has a shallower peak to fall from. Both are fixable, and neither means the drug has stopped working.

Drug Activity Timeline: What Happens Each Day

A weekly GLP-1 injection follows a predictable arc: absorption and peak in the first 1 to 2 days, a comfortable plateau through mid-week, then a steady decline to the weekly low just before your next dose. Side effects cluster near the peak, and hunger returns near the trough.

What Happens If You Miss An Injection?

Missing one dose is not an emergency, because a long half-life means meaningful drug remains for days. For semaglutide, take the missed dose within 5 days and then resume your normal day. For tirzepatide, take it within 4 days as long as your next dose is at least 3 days later. If you are outside that window, skip it.

Never double up to catch up. Two doses close together stack on each other and produce exactly the nausea and vomiting that gradual titration is designed to avoid.

After a longer gap, several weeks rather than several days, drug levels fall far enough that restarting at your full dose can feel like starting over. Your prescriber may restart you lower and titrate back up, which is a normal precaution rather than a setback.

• Semaglutide (Ozempic, Wegovy): take it within 5 days, otherwise skip it
• Tirzepatide (Mounjaro, Zepbound): take it within 4 days if the next dose is at least 3 days away
• Never double up: semaglutide doses must be at least 48 hours apart, tirzepatide at least 72 hours
• After a gap of several weeks, contact your prescriber before injecting your usual dose

Can You Inject A Day Early Or A Day Late?

A day either side of your usual injection day is generally fine and is explicitly allowed for both semaglutide and tirzepatide, as long as consecutive doses stay at least 48 hours apart for semaglutide, or 72 hours for tirzepatide. Because the half-life is measured in days, shifting by 24 hours barely changes your level. Consistency still beats convenience.

Changing your injection day permanently is also allowed, as long as you keep that minimum gap between doses: 48 hours for semaglutide, 72 hours for tirzepatide. Many people move their shot to a Friday or Saturday so that peak-day nausea lands on a day off rather than a workday.

Travel, Flying And Time Zones

Time zones do not meaningfully affect a drug with a multi-day half-life, so keep your usual injection day and inject at whatever local time is convenient. Carry pens in your hand luggage, never in checked baggage, where the hold can freeze them. A pharmacy label and your prescription make security straightforward.

Keep the medication cool in transit with a travel case and a cool pack, and avoid leaving it in a hot car or in direct sun. If a trip means you will miss your usual day entirely, shifting the injection a day or two either side is safer than skipping it, as long as doses stay at least 48 hours apart for semaglutide, or 72 hours for tirzepatide.

Storage: Refrigeration And Warm Medication

Store unopened GLP-1 pens in the fridge between 2 and 8 degrees Celsius, and never freeze them. A pen that has frozen must be thrown away even if it looks normal. Once in use, most pens can be kept at room temperature for a limited period, which is set by the manufacturer, so check your own leaflet.

Letting a cold pen sit out for 20 to 30 minutes before injecting makes the shot noticeably more comfortable, because cold liquid stings more going in. Never warm a pen deliberately with hot water or a microwave, and never use medication that looks cloudy, discoloured or has particles in it.

• Unopened pens: refrigerate at 2 to 8 degrees Celsius and never freeze
• In use: room temperature is allowed for a limited window, check your specific leaflet
• Discard any pen that has frozen, even if it looks fine
• Let a cold pen reach room temperature naturally before injecting to reduce sting
• Never use medication that is cloudy, discoloured or contains particles

Protein: Eat It When You Can Actually Eat

Aim for roughly 1.2 to 1.6 grams of protein per kilogram of body weight daily during active weight loss. The practical challenge on a GLP-1 is timing, not arithmetic: appetite is lowest right after your injection, so front-load protein in the window where eating is easiest and always put it first on the plate.

This matters because protein is what decides whether the weight you lose is fat or muscle. When appetite drops, protein is the first thing people under-eat, and low protein during rapid loss accelerates muscle loss, which lowers your metabolism and makes the weight easier to regain.

Exercise: Timing Your Training Around The Curve

Train hard in the middle of your week, roughly days 2 to 5, when early nausea has settled but drug levels remain high. Resistance training two to three times weekly is the highest-value exercise on a GLP-1, because it is the signal that tells your body to keep muscle while it sheds fat. Add moderate cardio for heart health.

The 24 to 48 hours right after an injection are the most likely to bring nausea and fatigue, so keep those days lighter: walking, mobility work, or an easy session rather than your heaviest lifts.

Hydration: The Habit That Fixes The Most Symptoms

Aim for about 30 to 35 ml of fluid per kilogram of body weight daily, which is roughly 2 to 3 litres for most adults, and more when nausea or vomiting is in play. GLP-1 medications dull thirst signals along with hunger, so dehydration creeps up without you noticing.

If you change one habit during your peak days, make it this one. Consistent hydration measurably reduces the two most common complaints on these drugs: headaches and constipation.

Common Questions About GLP-1 Drug Activity

Short, direct answers to the questions people ask most about how long their medication lasts, when to inject, and why appetite shifts through the week.