What are the side effects of Ozempic?
Semaglutide causes gastrointestinal complaints in one in four users, most commonly nausea; these typically resolve over time. Rarer signals such as optic nerve damage require medical follow-up, so this medication should always be used under the supervision of a doctor.
Gastrointestinal complaints are by far the most common side effects of semaglutide. Nausea has the highest share: one in five to one in four users in large clinical trials experienced it, compared with about one in twelve in the placebo group. Diarrhoea affects approximately one in ten users. Vomiting, stomach discomfort and constipation each occur in roughly 5 to 7% of users. These complaints are usually mild to moderate and often resolve over time, but they are the main reason people discontinue the medication.
Headache and a marked reduction in appetite were each reported in around 7 to 8% of users. Gallstones are a recognised risk across the entire class of similar medications to which semaglutide belongs. Patients with pre-existing diabetic eye damage need particularly close monitoring: a rapid drop in blood sugar can temporarily worsen the eye damage, especially in those who also use insulin.
A large analysis of more than 34,000 patients across 21 randomised trials found no increased risk of acute pancreatitis. Thyroid cancer also remained below 1% and did not differ from the control group, although study durations are too short for a definitive conclusion about such a rare event.
Two rarer signals deserve attention. An observational study found a markedly increased risk of a rare optic nerve condition (NAION) in semaglutide users, but cause and effect have not yet been established and the numbers involved are small. In addition, rare but serious skin reactions have been reported, including hypersensitivity reactions and a rare blistering disease. The medication must be discontinued if such reactions occur.
Serious adverse events in the broad sense (anything requiring hospitalisation or intensive medical attention) occurred in 7 to 25% of participants in clinical trials, depending on the patient population studied. That wide range illustrates how strongly the outcome depends on who is being treated. Regular medical monitoring is not a luxury with this medication.
This answer is based on two large randomised multicentre trials (including SUSTAIN 1), two systematic reviews of multiple trials, a large analysis of 21 trials with more than 34,000 patients, and additional reviews on specific side effects such as skin and eye problems. The evidence for gastrointestinal complaints is robust and consistent. The signal for optic nerve damage (NAION) comes from a single observational study without a demonstrated causal relationship and should be regarded as preliminary.