GLP-1 Side Effects: What the Trials and FDA Labels Actually Show

Most of what circulates about GLP-1 side effects is anecdote. The pivotal trials and the FDA-approved prescribing information are not — they are the two places a real percentage comes from, and this piece sticks to those.

The common ones, with real numbers

Gastrointestinal symptoms are the headline side effect of every drug in this class, and they are common enough that both FDA labels list them as the most frequent adverse reactions. On Zepbound's label, in the pivotal obesity trials: nausea in 20% of patients, diarrhea in 15%, vomiting in 11%, constipation in 6%, and lower rates of dyspepsia, abdominal pain, belching, reflux, bloating and gas⁴. Semaglutide's STEP 1 trial reported the same pattern of nausea and diarrhea as the most common events, described as typically transient and mild-to-moderate¹.

The number that actually measures severity is how many people quit the drug over it. In STEP 1, 4.5% of semaglutide participants discontinued treatment because of gastrointestinal adverse events, versus 0.8% on placebo¹. In SURMOUNT-1, discontinuation for adverse events rose with tirzepatide dose — 4.3% at 5 mg, 7.1% at 10 mg and 6.2% at 15 mg, against 2.6% on placebo². Read plainly: most people who start either drug tolerate it well enough to stay on it, and the minority who don't skews toward the higher doses.

Why the dose escalates instead of starting at target

Both labels prescribe a slow climb to the maintenance dose specifically to hold GI side effects down — Wegovy over roughly 16-20 weeks, Zepbound over a minimum of 20³⁴. Jumping straight to a "target" dose skips the reason the schedule exists. The full step-by-step numbers, plus what to do about a missed dose, are in GLP-1 dosage and titration schedules.

The warnings that aren't gastrointestinal

Both Wegovy and Zepbound carry the same boxed warning: in rodent studies, the drug caused thyroid C-cell tumors, and it is unknown whether that risk translates to humans — both are contraindicated in anyone with a personal or family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia syndrome type 2³⁴. Beyond the box, both labels warn on:

- **Pancreatitis** — watch for persistent or severe abdominal pain, sometimes radiating to the back, with or without nausea or vomiting³. Heavy alcohol use is an independent pancreatitis risk factor, which is why it's worth reading before, not after — see GLP-1 and alcohol. - **Gallbladder disease** — in Zepbound's trials, cholelithiasis was reported in 1.1% of drug-treated patients versus 1% on placebo, and cholecystitis in 0.7% versus 0.2%⁴. - **Hypersensitivity** — serious reactions including anaphylaxis and angioedema have been reported with both³⁴. - **Ileus** — added to the Wegovy label in 2023, after post-marketing reports³. - **Mood changes** — Zepbound's label specifically directs monitoring for new or worsening depression, suicidal thoughts or behavior, or unusual mood changes⁴.

None of these are common. They are on the label precisely because they are the ones worth knowing before day one rather than after a symptom shows up.

Two things that get reported anyway, but aren't on either label

Hair thinning and a loss of lean muscle alongside the fat are both widely reported by patients and increasingly studied, but neither is an FDA-labeled adverse reaction in the way nausea is — they are downstream of rapid weight loss itself, not a direct drug effect in the same sense. Both get the dedicated, sourced treatment they need in GLP-1 and hair loss and GLP-1 and muscle loss rather than a paragraph here.

When a side effect is a reason to call, not wait

Persistent severe abdominal pain, signs of an allergic reaction, or a new or worsening mood change are the three symptom patterns both labels single out as needing prompt medical attention rather than a "give it another week." Everything else on the common list — nausea, mild diarrhea, early constipation — is the pattern the titration schedule exists to manage, and it typically settles as the dose holds steady.

Choosing on this basis

If tolerability is the deciding factor between the two molecules, the trial-level discontinuation rates above are the most honest single number to compare, and our semaglutide vs tirzepatide breakdown walks the rest of that trade-off. If needles themselves are the issue rather than the drug, see injections vs oral GLP-1. And before choosing who dispenses either drug, how to tell if a GLP-1 provider is legitimate covers the operational side of the same due-diligence habit. Every claim on this page is cited above; the rest of the Desk's sourced evidence is collected in the research library.