GLP-1 Side Effects: What's Normal, What's Not, and How to Manage Them
GLP-1 medications are effective, but they're not without side effects. Nausea is the most common — and the one most likely to cause people to stop the medication before giving it a fair chance. Understanding what to expect, why it happens, and how to manage it can make the difference between sticking with the treatment and giving up in the early weeks.
The Most Common Side Effects
Gastrointestinal side effects account for the majority of complaints with GLP-1 medications. In the STEP trials for Wegovy, nausea was reported by about 44% of participants, vomiting by about 24%, and diarrhoea by about 30%. These numbers sound daunting, but they include any occurrence over the full trial period — most were mild and temporary, not ongoing or severe.
Nausea
The most common side effect, and the most manageable. It typically peaks in the day or two after injection and then fades. It's worst during dose escalation — particularly when moving to a higher dose — and tends to improve significantly once you've been at any given dose for a few weeks.
Practical things that help with nausea:
- Eat smaller meals more frequently, rather than large meals
- Eat slowly and stop before you feel completely full
- Avoid fatty, fried, or strongly spiced foods in the days after injecting
- Stay upright for at least an hour after eating
- Keep well hydrated — nausea is often worse when you're dehydrated
- Ginger (tea, supplements, or food) has mild anti-nausea properties that some people find helpful
- If nausea is severe, ask your prescriber about anti-nausea medication short-term
Vomiting
Less common than nausea, but it happens. The same dietary strategies that reduce nausea tend to reduce vomiting risk. Eating too much too quickly on GLP-1 medication is a common trigger — the stomach empties slowly, and eating past the point of fullness can cause discomfort that leads to vomiting. Most people find they naturally adjust their eating patterns once they're on the medication.
Constipation
Because GLP-1 medications slow gastric emptying and gut motility, constipation is quite common — reported in roughly 24% of Wegovy trial participants. Unlike nausea, which tends to improve over time, constipation can persist. Adequate hydration, increased dietary fibre (vegetables, legumes, wholegrains), and regular movement all help. Some people need a fibre supplement or occasional mild laxative. If it's persistent, mention it to your prescriber.
Diarrhoea
Diarrhoea is a less-discussed but common side effect, particularly in the early weeks. It often alternates with constipation as the gut adjusts to the medication. It usually settles with time. Stay hydrated and mention it to your prescriber if it's frequent or severe.
Fatigue
Some people report tiredness, particularly in the first few weeks. This is partly related to the reduced food intake — eating significantly less means less energy intake. Making sure you're eating enough protein and nutrients (even in smaller amounts) helps. Fatigue usually improves as the body adapts.
Less Common But Important Side Effects
Headaches: Relatively common in early weeks, often related to reduced food and fluid intake. Hydration and consistent meal patterns tend to help.
Hair thinning: Some people notice hair loss several months into treatment, typically related to significant and rapid weight loss (telogen effluvium) rather than a direct drug effect. It's almost always temporary. Adequate protein intake matters here.
Muscle loss: Rapid weight loss from any cause can include muscle loss alongside fat loss. Resistance exercise during your treatment period helps preserve lean muscle mass and is worth prioritising regardless of what you eat.
When to Contact Your Prescriber
Seek medical advice if you experience:
- Severe or persistent abdominal pain, particularly if it radiates to your back — this can indicate pancreatitis, a rare but serious complication
- Vomiting that's severe enough to prevent you eating or drinking
- Signs of low blood sugar (if you have diabetes): shaking, sweating, confusion, rapid heartbeat
- Rapid heart rate at rest that doesn't settle
- Vision changes
- Significant swelling in your neck area (possible thyroid concern — rare, but listed in prescribing information)
Do Side Effects Get Better Over Time?
For most people, yes. Nausea is typically worst in the first few weeks at any given dose and improves significantly once you've been at that dose for four to six weeks. Vomiting and diarrhoea follow a similar pattern. Constipation is the notable exception — it can persist and may need ongoing management.
The dose escalation schedule exists specifically to minimise side effects by giving your body time to adjust before increasing. If you're experiencing significant side effects, it's worth asking your prescriber whether you should slow down the escalation — spending eight weeks at a dose rather than four — rather than stopping the medication altogether.
Track your injections alongside your progress
Logging each injection with the date and dose helps you spot whether side effects are timing-related (worse in the 1–2 days after injection) or dose-related (worse since moving to a higher dose). That's useful information for your prescriber.
Open the free tracker →What Actually Causes the Nausea?
GLP-1 receptors exist not just in the gut, but in the brain stem — specifically in an area involved in regulating nausea and vomiting. The medication's effects on gastric emptying (slower stomach emptying) combined with direct central effects on the brain's nausea centres explains why GI side effects are so common. It's not an allergic reaction or a sign that something is wrong — it's the drug doing what it does. The good news is that the same mechanism that causes nausea is also responsible for the appetite suppression that drives weight loss.
Medical disclaimer: This article provides general information about common GLP-1 side effects and is not a substitute for medical advice. Side effect profiles vary between individuals and between medications. Always consult your prescriber if you experience significant or unexpected symptoms.