Gastrointestinal effects are the reason most people stop taking a GLP-1, and they are also the reason the dose is escalated slowly rather than started high. Here is what the pivotal trials recorded, and what the numbers do and do not tell you.
What was reported
In STEP 1, gastrointestinal effects — predominantly nausea and diarrhea — were the most common adverse events on semaglutide, typically transient and mild to moderate, and arising mostly during dose escalation.[1] SURMOUNT-1 recorded the same pattern for tirzepatide, with nausea, diarrhea, vomiting and constipation leading and most events mild to moderate.[2]
The figure worth anchoring on is not the incidence but the discontinuation rate. In SURMOUNT-1, adverse events led to treatment discontinuation in 4.3% to 7.1% of participants on tirzepatide against 2.6% on placebo.[2] In STEP 1 the corresponding figure was 7.0% against 3.1%.[1]
Read plainly: most people in these trials experienced side effects and most of them continued anyway. A small but real minority could not.
Why escalation exists
Both drugs start well below their maintenance dose — semaglutide at 0.25 mg, tirzepatide at 2.5 mg — and step up over months. That schedule exists specifically to limit the effects above. Moving faster than the labeled schedule tends to produce more of them, not faster results. The doses those schedules climb toward are the ones the trials measured.
This has a commercial consequence worth knowing. Because escalation is the normal path, a seller that re-prices at each dose step charges more precisely as you move toward the dose the trials studied — see sellers that hold one price.
The effects that are not gastrointestinal
The labeled warnings for this class include pancreatitis, gallbladder disease, kidney injury from dehydration secondary to vomiting, and a boxed warning regarding thyroid C-cell tumors observed in rodents. These are uncommon relative to the gastrointestinal effects and they are the ones that warrant contacting a prescriber rather than waiting out.
Persistent severe abdominal pain is the symptom most worth acting on promptly rather than attributing to ordinary nausea.
What these figures do not cover
Every rate above comes from trials of the branded products at labeled doses under trial supervision. Most sellers on this site supply compounded versions, which are not FDA-reviewed before dispensing, and some market low-dose or microdose protocols that sit outside any labeled schedule. The side effect profile of a protocol nobody has trialled is not something these numbers describe.
Tolerability is also what decides whether treatment continues at all, which matters for the result and for the bill — see the price board.
None of this is medical advice, and side effects are a prescriber conversation rather than a website one.