Short version: I took tirzepatide (Mounjaro) in two courses in 2026. The 2.5 mg starter dose took me from 96.4 kg to 90.2 kg by the end of March, then a dose cut, a femur fracture and a supply gap ended it. The second course, four Japanese 2.5 mg pens, got me to 86.2 kg, my lowest weight of the year. Each time the drug stopped, the weight came back within weeks.
Why do I call tirzepatide "GLP-2"?
It is my own shorthand. Semaglutide works on one receptor, GLP-1. Tirzepatide works on two, GIP and GLP-1. Retatrutide adds a third, glucagon, and people call it "GLP-3". So in my own notes tirzepatide became "GLP-2", the two-receptor drug.
That nickname collides with a real hormone. GLP-2 (glucagon-like peptide-2) is a gut hormone in its own right, and the drug that targets it, teduglutide, treats short bowel syndrome. It has nothing to do with appetite or weight. If you searched "GLP-2" for weight loss, this page is about tirzepatide.
What happened on the first tirzepatide course?
It worked, at the lowest dose there is. A clinic in Bangkok prescribed a 5 mg Mounjaro KwikPen at a 2.5 mg weekly start, and the first injection went in on 20 February 2026, a Friday evening, so I slept through any side effects. The pen is multi-dose, so 2.5 mg meant dialling part of a 5 mg dose: 30 clicks. That is my record of how it was prescribed and taken, not a method to copy.
| Period | Weekly dose | What happened |
|---|---|---|
| 20 Feb to 20 Mar | 2.5 mg (30 clicks) | Appetite quiet for about days 1 to 5 after each dose. 96.4 kg (16 Feb) to 92.0 kg (16 Mar) |
| 27 Mar to 3 Apr | about 1.67 mg (20 clicks) | Dose cut ahead of a race. 90.2 kg on 29 Mar, then 93.6 kg on 6 Apr |
| 12 and 16 Apr | about 1.25 mg (15 clicks) | Hunger back, small binges, worse with alcohol |
| 21 Apr | stopped | Femur fracture and surgery. Off the drug on purpose, to eat for the bone |
| early Jun | about 1.67 mg, once | Bridge dose, the last of the pen |
The lesson I took from the dose cut is the one the rest of this story keeps repeating: the drug was covering for the appetite, not fixing it. Ten days after stepping down I was 3.4 kg heavier than at the end of the five-week block. The full week-by-week numbers are in my GLP-1 log.
After the bike crash I stayed off it from April to June to give the femur the calories it needed. When I wanted back on, the pen was empty and no clinic in Saigon that I contacted stocked tirzepatide. On 29 June I moved to semaglutide (Ozempic) because that was what I could get.
Why did I go back to tirzepatide in August?
Because more semaglutide did not help. Between July and August I stepped the Ozempic dose up four times, and hunger still came back around day 6 every week. When four dose increases move nothing, the next thing to change is the molecule, and tirzepatide was the one I had been on when the first block worked.
What is it like to use the Japanese Mounjaro Ateos pen?
It is simpler than the KwikPen and less flexible. The Japanese box (マンジャロ皮下注 2.5mg アテオス) held two single-use autoinjectors, 2.5 mg in 0.5 mL each, marked for once-weekly use as the starting dose. I had four. There is nothing to dial: press, wait, done. The box carries the Lilly logo next to Mitsubishi Tanabe Pharma, because Eli Lilly Japan holds the authorisation and Mitsubishi Tanabe co-promotes it. In Japan the same molecule is sold as Mounjaro for type 2 diabetes and as Zepbound for obesity.
With a fixed dose, timing is the only lever left, and I used it more than I should have.
| Dose | Date | Days since previous | Why that day |
|---|---|---|---|
| 1 | 29 Aug | 6 (after the last Ozempic) | One day early: the semaglutide effect had worn off and I binged on carbs the evening before |
| 2 | 3 Sep | 5 | Three days early, after another evening binge, while sick and about to fly |
| 3 | 10 Sep | 7 | On schedule |
| 4 | 15 Sep | about 5.6 | Early again, the last pen I had |
Tirzepatide has a half-life of about five days, according to the US prescribing information, which also sets 2.5 mg as the starting dose and steps up in 2.5 mg increments after at least four weeks. A dose every five days stacks on top of the last one instead of replacing it. Spacing is a call for the prescriber, and I made those calls without one.
The scale moved anyway: 89.7 kg on 29 August, 86.2 kg fasted on 17 September, two days after the last pen. That was the lowest reading of my whole year.
The resupply came through as well. In Bangkok in early September I picked up two 5 mg Mounjaro KwikPens, the dialable kind from my first course. At 0.0835 mg per click, 30 clicks is 2.5 mg, so the dose became a lever again. Unlike the Ateos pens, KwikPens have to be kept in the fridge.
What happened when I stopped tirzepatide?
The weight came back, for the third time. I was in Germany when the last Japanese pen was used and flew on to the US. The two KwikPens from Bangkok would have let me carry on, and I did not start them. That was my choice, not an empty box, though there was no taper and no clinical decision behind it.
- 17 September (2 days off): 86.2 kg
- 22 September (7 days off): 88.1 kg
- 4 October (19 days off): 88.9 kg
Some of that is glycogen, water and two long flights, but the trend kept pointing up. In SURMOUNT-4, people switched from tirzepatide to placebo after 36 weeks regained 14% of their body weight over the next year. My dose was far lower and my course far shorter, so the comparison is loose, but the first weeks of my curve point the same way. The longer write-up is in the October update of my GLP-1 log.
Did tirzepatide work for me?
Yes, while I was on it, and at a sixth of the 15 mg maximum dose. In SURMOUNT-1, adults with obesity lost 15.0% of body weight on 5 mg and 20.9% on 15 mg over 72 weeks, against 3.1% on placebo. I am a lean athlete, not that population, and I never went past the starter dose. Two things did not work:
- Day 5. On both courses, appetite came back before the week was out. On the Japanese pens, that is why the intervals got short.
- The habits. The plan was to build eating habits in the quiet weeks that would hold after stopping. Each time I stopped, the weight came back, so they did not hold.
I left tirzepatide twice, never because it failed: the first time I could not get it, the second time I stopped with pens still in hand. On 20 October I started retatrutide instead, to see whether a third receptor moves the day-5 wall. That is a different story, with different risks, and it is written up separately.
What would I tell another athlete about tirzepatide?
- Plan the supply before the first dose. My first course ended because the pen ran out, not because a doctor or I decided.
- Expect the weight back when it stops, and decide in advance what replaces the drug.
- Watch performance, not just the scale: strength, resting heart rate, session quality.
- If you compete in tested sport, check the current WADA Prohibited List yourself.
- Keep the prescriber in the loop on timing, which I did not always do.
Disclaimer
This is my own log, written up for transparency. It is not medical advice and not a recommendation to use tirzepatide or any other drug. Tirzepatide is a prescription medicine; talk to your doctor. I do not sell, link to, or take money from any pharmacy or drug maker.
Frequently Asked Questions
Is tirzepatide a GLP-2 drug?
No. Tirzepatide acts on two receptors, GIP and GLP-1, and I call it "GLP-2" only as shorthand for "two receptors", the same way people call retatrutide "GLP-3". GLP-2 is a separate gut hormone. The drug that actually targets it is teduglutide, used for short bowel syndrome, and it has nothing to do with weight loss.
How much weight did you lose on the tirzepatide starter dose?
From 96.4 kg on 16 February 2026 to 90.2 kg on 29 March, on 2.5 mg a week. The second course, four 2.5 mg doses from 29 August, took me from 89.7 kg on 29 August to 86.2 kg on 17 September. Both are one athlete in heavy training, and the first kilos of any deficit are partly water and glycogen.
How long did one tirzepatide dose control your appetite?
About five days. On the first course the starter dose held for roughly days 1 to 5 after each Friday injection. On the Japanese pens I took two doses early after evening binges, and the four intervals were 6, 5, 7 and about 5.6 days instead of 7.
What is the Japanese Mounjaro Ateos pen?
A single-use, fixed-dose autoinjector. The box I used held two 2.5 mg / 0.5 mL pens marked for once-weekly use as the starting dose. Eli Lilly Japan holds the Japanese authorisation and Mitsubishi Tanabe co-promotes it. Unlike a KwikPen, there is nothing to dial, so the only thing you can change is the timing.
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