Semaglutide side effects and safety context

Semaglutide's most common side effects are gastrointestinal — nausea affected 44% of participants in the pooled Wegovy trials (vs 16% on placebo), followed by diarrhea (30%) and vomiting (25%). These are usually worst during dose escalation and fade with time; about 4% of trial participants stopped the drug because of adverse reactions. Behind the common effects sit a boxed warning about thyroid C-cell tumors seen in rodents and a short list of rarer but serious risks: pancreatitis, gallbladder disease, dehydration-related kidney injury, and worsening of diabetic retinopathy (Wegovy prescribing information).

The GI profile follows directly from the mechanism — slowed gastric emptying and central appetite suppression. Rates scale with dose. From the labels:

• Wegovy (2.4 mg weekly), vs placebo: nausea 44% vs 16%, diarrhea 30% vs 16%, vomiting 25% vs 6%, plus constipation, dyspepsia, abdominal pain, and reflux at lower rates (Wegovy label).
• Ozempic (diabetes doses), vs placebo: nausea 15.8% at 0.5 mg and 20.3% at 1 mg vs 6.1%; vomiting, diarrhea, abdominal pain, and constipation are the other reactions reported in ≥5% of patients (Ozempic label).

This is why every labeled product titrates from a 0.25 mg starter dose over months rather than beginning at the maintenance dose — the ramp exists for tolerability, as covered on the dosing education page. In the Wegovy program, permanent discontinuation because of a gastrointestinal reaction was 4.3% vs 0.7% on placebo, and severe gastrointestinal reactions were reported in 4.1% vs 0.9% — most people tolerate the drug well enough to stay on it.

Semaglutide's boxed warning comes from rodent studies: it caused thyroid C-cell tumors in rats and mice at clinically relevant exposures. Whether this applies to humans is unknown — medullary thyroid carcinoma is rare and the rodent mechanism may not translate — but out of caution the drug is contraindicated in anyone with a personal or family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia syndrome type 2 (Ozempic label). It is also contraindicated after a prior serious hypersensitivity reaction to semaglutide.

• Acute pancreatitis — observed with GLP-1 receptor agonists including semaglutide; labels direct discontinuation if pancreatitis is suspected.
• Gallbladder disease — gallstones and cholecystitis occur more often on treatment, partly a consequence of rapid weight loss itself.
• Kidney injury from dehydration — severe vomiting or diarrhea can cause volume depletion and acute kidney injury, a mechanism distinct from semaglutide's protective kidney effects in the FLOW trial.
• Diabetic retinopathy complications — in SUSTAIN-6 (3,297 patients with type 2 diabetes), retinopathy complications were significantly more frequent with semaglutide (hazard ratio 1.76), likely tied to rapid glucose lowering; patients with existing retinopathy are monitored (NEJM 2016).
• Hypoglycemia — uncommon with semaglutide alone because insulin release is glucose-dependent, but a real risk in combination with insulin or sulfonylureas.

For the class-wide long-term picture, see are GLP-1 peptides safe long term?

Every number above comes from the approved, quality-controlled product. Unapproved semaglutide adds hazards the trials never measured. The FDA has received multiple adverse-event reports — some requiring hospitalization — linked to dosing errors with compounded injectable semaglutide, from patients and even clinicians miscalculating doses. It also warns that some compounded products contain salt forms (semaglutide sodium, semaglutide acetate) that are different active ingredients from the approved drug, with unestablished properties (FDA). Raw "research" vials sit further outside oversight still: purity, sterility, and actual peptide content are unverified, and dosing depends on the buyer's own reconstitution math.