GLP-1 side effects - what is common, manageable, and when to contact us

Roughly 70 percent of patients starting GLP-1 medication experience some side effects in the first month. Almost all are manageable, most settle within 6-12 weeks, and a very small proportion require pausing the medication or switching drug. This article lists what to expect, what to manage yourself, and when to get in touch with us quickly.

Common and usually manageable

Nausea (60-70 percent in first month): worst in the 12-48 hours after injection. Management: smaller meals, avoid fatty or ultra-processed food after injection, ginger tea, anti-emetics available on request.

Constipation (50-60 percent): the quiet side effect that catches people out. Management: drink 2.5 litres of water daily, fibre (psyllium, flaxseed, oats, vegetables), daily walking, laxatives short-term if needed.

Reflux / heartburn (30-40 percent): slowed gastric emptying makes food sit longer. Management: smaller meals, stay upright for 2 hours after eating, raise head of bed, omeprazole short-term if persistent.

Fatigue (20-30 percent in first weeks): usually transient. Management: rest, hydration, regular sleep schedule.

Headaches (15-20 percent): often from dehydration or low food intake. Management: regular fluids, do not skip meals entirely.

Injection-site reactions (10 percent): small red mark. Management: rotate injection sites, leave at least an inch between weekly injections.

Dry mouth and altered taste (15-20 percent): often transient. Management: hydration, good dental hygiene.

Mild hair thinning (10-15 percent, usually from month 3): related to rapid weight loss rather than the drug specifically. Recovers after weight stabilises.

Most of these settle by week 6-8. We are happy to support symptom management at any point; unlimited messaging is included in your ongoing care subscription.

Less common but worth knowing

Gallstones (1-2 percent): rapid weight loss is a recognised gallstone trigger, independent of the drug. Management: ursodeoxycholic acid 300mg daily during rapid loss phase if you are prone, or if ultrasound shows sludge. We screen for this.

Mood changes (1-3 percent): low mood, anxiety, rarely suicidal ideation. The data are mixed on whether this is a drug effect or a confounder (rapid weight loss changes body image, social dynamics, food-reward). We screen at every review and will pause the medication if needed.

Reduced lean muscle mass (variable, typically 25-30 percent of total weight lost is lean mass): more pronounced in older patients and patients with low protein intake or no resistance exercise. Management: protein intake 1.2-1.5g/kg/day, resistance training 2-3 times per week.

Kidney function changes: rare, usually associated with dehydration from severe vomiting. Resolves with fluid correction. We monitor kidney function on your biannual bloods.

Rare but important - contact us today

These are genuinely uncommon (fewer than 1 percent of patients) but clinically serious. Any of the following, message us or contact NHS 111 the same day:

Severe abdominal pain, particularly upper abdominal pain radiating to the back (possible pancreatitis). Stop the medication, go to A&E, do not wait for us to respond.

Persistent vomiting for more than 24 hours, unable to keep fluids down. Dehydration risk.

Jaundice (yellow skin or eyes): possible gallstone blocking the bile duct. A&E.

Severe allergic reaction: rash, wheezing, face/lip swelling, difficulty breathing. 999.

Visual disturbances, particularly in patients with diabetic retinopathy. Associated with rapid glycaemic change rather than the drug.

Unexplained severe weakness, dizziness on standing, confusion, or fainting: likely dehydration but worth immediate review.

Blood in stool or vomit: rare but always worth checking.

Specific to patients with diabetes

If you are on insulin or sulfonylurea alongside GLP-1 medication, hypoglycaemia is a real risk because the GLP-1 reduces appetite while the other drugs continue to lower glucose. We adjust diabetes medication when we start GLP-1 in diabetic patients and monitor closely.

Specific to patients with type 1 diabetes

Type 1 patients should not be on GLP-1 medication for weight management. We do not prescribe for this group; there are specific risks (diabetic ketoacidosis) that make it inappropriate.

How we monitor you

Monthly review: weight, BMI, blood pressure, heart rate, side effect log, mood screen, food intake review.

Quarterly: body composition if appropriate, comprehensive side effect review.

Biannually: full metabolic bloods (HbA1c, liver function, kidney function, lipids, full blood count, thyroid if indicated).

Annually: comprehensive metabolic and cardiovascular review.

Adjustments are made based on what we find, not a fixed schedule.

What to do if you are getting bad side effects but want to continue

Three options in order of preference:

Slow the titration. If you are at dose X and nauseated, stay at X for longer rather than stepping up. Four to six weeks at a dose is usually enough for side effects to settle.

Drop the dose. If you cannot tolerate the current dose even after several weeks, step back down one dose. You keep most of the weight-loss benefit at the lower dose.

Switch medication. If Wegovy is not tolerated, switch to Mounjaro (or vice versa). Some patients who cannot tolerate one tolerate the other well.

Pause entirely. If nothing is working, pause the medication and reassess. Most side effects resolve within a week of stopping. You can restart at a lower dose after a pause.

The honest bottom line

Side effects are real but usually manageable. Most settle in the first 6-8 weeks. A small proportion of patients (5-10 percent) cannot tolerate GLP-1 medication at any dose and stop entirely. Everyone else adapts and the treatment becomes routine.

Ready to start?

If this article has made you think medication might help, the next step is a short consultation with one of our weight-management doctors.

Begin your consultation at this link. Online in 30 minutes, or in person at Westfield London.