GLP-1 side effects, symptom by symptom

20-44% of trial participants (Wegovy 44%, Ozempic 20%, Mounjaro 22%, Zepbound 28%, Saxenda 39%)

Nausea

The single most common GLP-1 side effect. Peaks weeks 1-4 after each dose increase, resolves for >80% of patients by week 8 at stable dose. Mitigation: smaller meals, hydration, avoid greasy food.

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10-19% of trial participants

Constipation

Common, manageable. Slower gut motility + reduced food/fiber intake creates predictable constipation. Mitigation: fiber 25-35g/day + 80oz fluids + magnesium oxide if needed.

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~3% of trial participants vs 1% placebo (STEP-1)

Hair loss (telogen effluvium)

Not a GLP-1-specific effect. Rapid weight loss of any kind triggers telogen effluvium in some people ~3 months in. STEP-1 reported 3% incidence vs 1% placebo. Resolves 6-12 months after weight stabilizes.

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~7-13% during titration weeks

Fatigue + low energy

Common in early titration from reduced calorie + carb intake. Persistent fatigue past week 8 warrants thyroid, ferritin, B12 labs.

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~2-3% of trial participants vs 0.5-1.2% placebo

Gallbladder issues

Rapid weight loss elevates gallstone risk regardless of drug. STEP-1 reported 2.6% incidence vs 1.2% placebo. Mitigation: gradual weight loss + maintained dietary fat.

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≤0.2% per year in cardiovascular outcome trials

Pancreatitis claim

Rare known risk (≤0.2%/year in trials). Severe steady abdominal pain that radiates to the back = ER visit. Personal pancreatitis history is a relative contraindication.

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Reddit-aggregated reports suggest 10-25% of patients experience this at some point; not formally tracked in trial data

Sulfur burps

Rotten-egg-smelling belching from slow gastric emptying + protein fermentation. Reddit-popular but not in the FDA label. Mitigation: smaller protein portions, antacids, ginger.

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Not formally tracked; patient-reported in 5-15% of online corpora

Brain fog + cognitive changes

Reported but inconsistent. Likely from rapid calorie restriction + dehydration during titration. Resolves at stable dose; persistent symptoms past week 8 warrant workup.

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Adjusted hazard ratio ~4.28 (T2D) and ~7.64 (obesity) vs non-GLP-1 controls in the 2024 Hathaway et al. cohort. Absolute incidence remains rare: ≤0.05% of users over 3 years in the studied population.

NAION (sudden vision loss)

Non-arteritic ischemic optic neuropathy — sudden painless vision loss in one eye. A 2024 Mass Eye and Ear cohort found ~4× higher rate in T2D semaglutide users vs non-GLP-1 controls; FDA is monitoring. Rare in absolute terms but worth knowing the warning signs.

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