The common ones
Gastrointestinal effects dominate: nausea, vomiting, diarrhoea, constipation, abdominal pain, indigestion, burping and reflux. In the major trials these affected a large proportion of participants and were the most frequent reason for discontinuation.
They are typically worst in the days after a dose increase and tend to ease as your body adjusts to a given dose. This is exactly why titration schedules are slow and why a provider who escalates faster than the labelling is doing you no favours.
Fatigue, dizziness, headache and hair thinning are also reported. Hair shedding is generally attributed to rapid weight loss rather than the drug itself, and is usually temporary.
The serious ones
These are uncommon, but you should know the warning signs rather than discovering them at two in the morning.
- Pancreatitis — severe, persistent abdominal pain, often radiating to the back, with or without vomiting. Stop and seek medical attention.
- Gallbladder disease — pain in the upper right abdomen, fever, jaundice, clay-coloured stools. Rapid weight loss raises gallstone risk generally.
- Acute kidney injury — usually secondary to dehydration from prolonged vomiting or diarrhoea. Persistent GI symptoms are a reason to contact a clinician, not to push through.
- Hypoglycaemia — a significant risk if you also take insulin or a sulfonylurea. Those doses often need adjusting.
- Ileus and delayed gastric emptying — severe bloating with an inability to pass stool or gas warrants urgent assessment.
- Serious allergic reaction — swelling of the face, lips or throat, or difficulty breathing, is an emergency.
The boxed warning
Both drug families carry a boxed warning relating to thyroid C-cell tumours observed in rodent studies. Whether this translates to human risk has not been determined.
The practical consequence is the contraindication: these medications should not be used by anyone with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia syndrome type 2.
Anaesthesia and surgery
Because these drugs slow gastric emptying, there is a recognised concern about food remaining in the stomach during sedation, which raises aspiration risk.
Tell any surgeon, anaesthetist, dentist or endoscopist that you are taking a GLP-1, well in advance. Professional guidance on whether and when to hold a dose has been evolving, so this needs to be a conversation with the team performing the procedure rather than a rule you apply yourself.
Muscle loss is a real consideration
Weight lost rapidly from any cause includes lean tissue alongside fat. Because these medications can reduce intake substantially, protein intake and resistance training matter more than usual.
Very few telehealth programmes provide meaningful support on this, and it is one of the things worth asking about before choosing one.
Practical things that help
None of this is medical advice and none of it substitutes for talking to your prescriber, but these are the measures most commonly suggested for managing the common effects.
- Smaller portions, eaten more slowly, stopping at the first sign of fullness
- Reducing fatty and fried foods, which tend to sit worst
- Steady fluid intake — dehydration drives several of the more serious complications
- Fibre and, if advised, a stool softener for constipation
- Asking about staying longer at a tolerated dose rather than escalating on schedule
Mental health
Reports of mood changes and suicidal ideation in people taking these medications prompted regulatory review on both sides of the Atlantic. Reviews to date have not established a causal link, and monitoring guidance remains in place.
If your mood changes after starting, that is worth reporting to a clinician rather than dismissing. In the United States you can also report a side effect directly to the FDA through MedWatch.