Quick Answer
The most common side effects are nausea, diarrhoea, constipation, and vomiting. These are usually mild to moderate and tend to improve as your body adjusts over the first few weeks.
Detailed Answer
The most common side effects of GLP-1 weight loss medications (Mounjaro, Wegovy, and Saxenda) are gastrointestinal and include nausea, diarrhoea, constipation, vomiting, abdominal pain, and decreased appetite. These side effects are reported in clinical trials across all three medications.
Nausea is the most frequently reported side effect, occurring in over 20% of participants in the SURMOUNT-1 trial (Mounjaro) and the STEP 1 trial (Wegovy). It is typically most noticeable during the initial weeks of treatment and during dose escalation periods.
The gradual dose titration schedule — starting low and increasing every 4 weeks — is specifically designed to minimise these effects. Most patients find that gastrointestinal side effects improve significantly after the first 4–8 weeks as their body adjusts to the medication.
Practical strategies that help manage side effects include eating smaller, more frequent meals; avoiding greasy, spicy, or heavy foods; staying well hydrated; and eating slowly. Your prescriber may also suggest adjusting the rate of dose escalation if side effects are troublesome.
Less common but more serious potential side effects include pancreatitis, gallbladder problems, and changes in mental health. The MHRA has issued specific guidance on GLP-1 medications and the risk of pancreatitis. Any severe or persistent symptoms should be reported to your prescriber immediately.