GLP-1 / fat loss
Tirzepatide Side Effects: What 20 Weeks Taught Me
What tirzepatide actually feels like week to week, and why most of the trouble is front-loaded.
The quick version
- Dual-agonist GLP-1, and one of the few peptides here that is FDA approved.
- Up to about 22.5 percent body weight lost over 72 weeks in trials.
- Most side effects are a first-month tax. Climb the dose slowly.
What it is
Tirzepatide is an injectable peptide that acts on two receptors, GLP-1 and GIP, so people call it a dual agonist. Unlike most peptides in this space, it is fully approved. The FDA cleared it for type 2 diabetes (Mounjaro) and for weight loss (Zepbound).
What the research shows
In the SURMOUNT-1 trial, people without diabetes lost up to about 22.5 percent of their body weight over 72 weeks at the 15 mg dose (Jastreboff et al., 2022). The most common side effects were stomach ones: nausea, diarrhea, constipation, and vomiting. Most were mild to moderate and showed up mainly while the dose was going up.
What it felt like
My first month was the hardest by far. Nausea after the shot, food going cold on the plate because I was full at half a meal, and one rough night of reflux when I ate too late. By around week 6 my body settled and the appetite drop felt smooth instead of jarring. Burping and mild constipation were the two that hung around longest.
Dosing reality
The label starts at 2.5 mg per week as a starter dose, not a treatment dose, and steps up no faster than every 4 weeks. I held each step an extra week when a dose felt strong. Slower titration traded a little time for a lot less nausea.
The one mistake to avoid
The common mistake is treating 2.5 mg like a dose that should melt fat. It is there to let your gut adjust. Expecting big loss in week one just leads people to climb too fast and feel awful.
Bottom line
Tirz is the rare approved option here, and the side effects are mostly a first-month tax. Climb slowly and the back half is much easier.
Sources
Reminder: I am not a doctor and this is not medical advice. It is my own notes and reading of the research. Peptides sit in a legal grey area, research-grade is a real category, and it is on you to verify your own compliance. Talk to a qualified professional before you start anything, especially if you take other medication.