Three clocks running at once
Side effects on these drugs sit on three separate timescales, and confusing them is why the same symptom gets three contradictory explanations.
The weekly cycle
Where you are between shots. Peak about a day after injection, trough at the end of the week. Symptoms that track this clock repeat on the same day of the week, every week.
The dose step
The fortnight after an increase, when the level is climbing towards a new plateau. Symptoms that track this clock show up after each increase and fade as the step settles.
The whole treatment
Months. Some things ease as the body adapts; some, like constipation or hair shedding, are downstream of eating far less and follow the weight rather than the dose.
Once you know which clock a symptom is on, you know what to do about it – wait, plan around it, or raise it at your next appointment. Guessing which clock it is from memory almost never works, which is the entire argument for writing it down.
Days one to three after a shot
This is the peak window. A subcutaneous injection takes roughly a day to reach its maximum, and for most people the strongest effects sit in the 24 to 72 hours afterwards.
- Nausea is the most common, usually mild to moderate and heaviest on day one or two.
- Early fullness – a few mouthfuls in and you are done. This is the effect doing what it does, and it catches people out at planned meals.
- Reflux and sulphur burps, often linked to eating a large or fatty meal near the peak.
- Fatigue, sometimes with reduced appetite for anything at all including water, which then makes everything else worse.
- Injection site reactions, usually within a day and usually minor. Which sites are worse is highly individual – see injection sites.
A pattern worth noticing: many people find shifting their injection to an evening changes how the first day feels, because the peak lands overnight. Whether that helps you is exactly the kind of thing your own log can answer in a month.
Days four to seven
The level is falling. On tirzepatide, with a five-day half-life against a seven-day gap, this drop is steep enough that many people feel it clearly: appetite returning, food noise coming back, energy improving. On semaglutide the week is flatter and the change is subtler.
This is also where constipation tends to show up, because it lags the eating pattern rather than the level. If you ate very little at the start of the week, the consequence arrives at the end of it.
The most common misreading in this window is treating the return of appetite as the medication “stopping working”. It has not stopped; you are at the trough. The difference between those two explanations is enormous and a level curve settles it instantly. See tirzepatide and semaglutide half-lives.
The fortnight after a dose increase
Every step up restarts the adjustment. The first one to two weeks of a new dose are where side effects concentrate, and for most people each subsequent step is easier than they feared – although some steps have a reputation, and 1.7 mg on semaglutide and 10 mg on tirzepatide come up repeatedly.
Because the level takes four to five weeks to plateau, a new dose is still rising for the whole month. That is why the standard ladder holds each step for four weeks and why judging a dose in week two is judging something that has not arrived yet. Dose schedules and titration has the ladders for every product.
Months in
Some things do not follow the dose at all.
- Constipation usually tracks how little you are eating and drinking rather than the level, and it tends to persist as long as intake stays low.
- Hair shedding, when it happens, typically appears a few months after rapid weight loss rather than with the drug itself, and it follows the loss rather than the dose.
- Fatigue and low energy may be about protein and total intake rather than the medication, which is why nutrition tracking is worth having in the same app.
- Muscle loss is a real risk of any rapid loss. Tracking lean mass rather than only weight is how you see it.
These are the ones people miss, because by the time they appear nobody is still linking them to a treatment that started months ago. A log that goes back to day one is what makes the connection visible.
When timing stops being the answer
Some symptoms are not a timeline question. Severe or persistent abdominal pain, repeated vomiting, signs of dehydration, or anything that frightens you belongs with a clinician now, not in a pattern to be observed over six weeks. Nothing in this guide, and nothing in Penwise, is a substitute for that call.
How to track it so the pattern actually shows
Three habits make the difference between a log and a diary.
- Record the time, not just the date, of every shot. A Monday morning and a Monday night injection are twelve hours apart on the curve, which is enough to move where a symptom lands.
- Score severity rather than noting presence. “Nausea” every week tells you nothing. Nausea at 4, then 2, then 1 across three weeks at the same dose tells you it is settling.
- Log on the quiet days too. A pattern is made of the days a symptom was absent as much as the days it was there.
Penwise tracks twenty named side effects on a 0 to 5 scale and nine sliding measures – appetite, fullness, food noise, sweet, salty and fatty cravings, energy, mood and sleep – against the dose and the time of every shot. The Insights tab looks for which day after a shot a symptom tends to land, and only says so when there is enough of your own history behind it. It stays quiet rather than inventing a pattern from three data points, which is the difference between a finding and a horoscope.
Find your own pattern, not the average one
Twenty side effects, nine measures, timed against your actual doses. Free core tracker, no account.
Common questions
When are GLP-1 side effects worst?
How long does nausea last on Mounjaro or Ozempic?
Why do I feel fine for three days and hungry by day six?
Do side effects come back at every dose increase?
Is constipation caused by the drug or by eating less?
Can an app tell me whether a symptom is from my medication?
Sources
Every factual claim on this page traces to the product information a regulator publishes, or to a published trial. Links open on the site of whoever issued the document.
- Mounjaro: EPAR product information European Medicines Agency
- Wegovy: EPAR product information European Medicines Agency
- Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1) New England Journal of Medicine, 2022
- Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1) New England Journal of Medicine, 2021
Penwise is a tracking and education tool, not a medical device. Nothing on this page is medical advice, a diagnosis or a recommendation to start, stop or change a dose. Dosing decisions belong to you and your clinician. Ozempic, Wegovy, Rybelsus, Saxenda and Victoza are trademarks of Novo Nordisk; Mounjaro, Zepbound and Trulicity are trademarks of Eli Lilly. Penwise is not affiliated with, endorsed by or connected to either company.