What Are the Risks of Weight Loss Injections?

Because these medicines act on GLP-1 (and, in the case of tirzepatide, also GIP) receptors involved in appetite regulation and digestion, the digestive system is most often affected. According to NHS guidance, side effects reported with GLP-1 receptor agonists include:

• Nausea and vomiting — often most noticeable after a dose increase; many people find this settles as the body adjusts
• Diarrhoea or constipation — both are commonly reported, sometimes alternating
• Indigestion, reflux, and bloating — slower stomach emptying (a known effect of this medicine class) can contribute
• Fatigue and headache — particularly in the early weeks
• Injection-site reactions — redness, itching, or mild swelling where the pen is administered

Less Common but Serious Risks

A smaller number of people experience more serious effects. These include:

• Pancreatitis — severe, persistent abdominal pain that radiates to the back is a warning sign; seek medical attention promptly if this occurs
• Gallbladder problems — including gallstones, which may become more likely with significant or rapid weight change
• Rapid heart rate (tachycardia) — reported by some users
• Mood changes — some people report low mood or, rarely, thoughts of self-harm; contact your healthcare professional if you notice changes in your mental health
• Hypoglycaemia — particularly relevant for people taking other diabetes medicines alongside their injection

These medicines carry a black triangle (▼) designation in some countries, meaning they are subject to additional monitoring. If you experience a suspected side effect, report it through your national reporting scheme (such as the UK Yellow Card scheme) and speak with your prescriber or doctor. You can read more about the effects of weight loss injections in our dedicated guide, or visit our overview of what weight loss injections are for broader background.

One of the most practical things you can do when taking a weekly injection is keep a consistent log of how you feel — not to replace clinical advice, but to build a clear picture you can bring to your appointments. GLP Trackr's side-effect tracker lets you record symptoms as they happen, noting their severity and timing relative to each dose. Over weeks, patterns often emerge: perhaps nausea peaks on day two after each injection, or fatigue is linked to lower protein intake. Having that data makes conversations with your prescriber far more productive.

The app also includes injection-site tracking with rotation suggestions, which is directly relevant to one of the most preventable risks — injection-site reactions. Consistently injecting into the same spot can cause lipohypertrophy (a build-up of fatty tissue) and affects how the medicine is absorbed. GLP Trackr logs each site you use — abdomen, thigh, or upper arm — and suggests your next rotation automatically, so you are not relying on memory alone.

If you notice any symptom that is severe, persistent, or worrying — particularly abdominal pain, significant mood changes, or signs of an allergic reaction — contact your healthcare professional promptly. GLP Trackr is designed to help you record and reflect, not to replace clinical judgement. Any decision about your dose or whether to continue the medicine belongs with your prescriber, who knows your full medical history.

For a broader look at managing your treatment, visit our weight loss injections hub.

Start logging your symptoms and injection sites today — download GLP Trackr free on the App Store or Google Play and explore everything with a 7-day free trial.