Weight loss injections such as Mounjaro and Wegovy are now common in the UK, but many people still have important safety questions. Are weight loss injections safe? What side effects are normal? And how can you tell whether the medication is legitimate?

The evidence is reassuring when these medicines are prescribed correctly. Licensed GLP-1 weight loss injections have been tested in large clinical trials and monitored in real-world use. When supplied by a regulated UK provider after a proper clinical assessment, they have a well-understood safety profile. The bigger risk is often unregulated access, counterfeit pens, or treatment without medical supervision.

What Clinical Trials Show

Mounjaro, the brand name for tirzepatide, and Wegovy, the brand name for semaglutide, were both tested in major phase 3 trials before approval. SURMOUNT-1 studied tirzepatide in 2,539 adults with obesity, while STEP-1 studied semaglutide in 1,961 adults with overweight or obesity.

In SURMOUNT-1, serious adverse events were similar between tirzepatide and placebo. The most common side effects were gastrointestinal, including nausea, diarrhoea and constipation. Most were mild to moderate.

Semaglutide also has strong long-term safety data. The SELECT trial followed 17,604 adults with obesity and cardiovascular disease for nearly 40 months. Semaglutide reduced major cardiovascular events by 20% compared with placebo, and serious adverse events were less frequent in the semaglutide group. "

In SURMOUNT-1, serious adverse events with tirzepatide were similar in rate to placebo. In a systematic review of more than 40,000 adults across 22 trials, risks of pancreatitis and serious adverse events were similar between GLP-1 medications and placebo.

Jastreboff AM et al. SURMOUNT-1. NEJM, 2022. Systematic review: AJMC, 2025.

What Side Effects Are Most Common?

The most common side effects of GLP-1 weight loss injections are nausea, diarrhoea, constipation and sometimes vomiting. These happen because the medication slows gastric emptying, which also helps reduce appetite.

For most people, side effects are strongest when starting treatment or increasing the dose. Supervised real-world data shows they often improve over time. In one cohort, nausea fell from 35% to 8% over 12 months. This is why monitoring matters: treatment can be adjusted based on tolerance.

Rare but known risks

Rare risks include gallbladder problems and acute pancreatitis. These are known risks that prescribers should assess and monitor. The MHRA also reviewed concerns around depression and suicidal ideation. In 2024, it concluded that available evidence did not support a causal link between GLP-1 medicines and depression, suicidal thoughts or suicide.

These medicines are not suitable for everyone. People with a personal or family history of medullary thyroid carcinoma, Multiple Endocrine Neoplasia syndrome type 2, pancreatitis, pregnancy or breastfeeding may not be suitable. People taking diabetes medication may also need closer monitoring. "

MHRA review in September 2024 concluded that available data do not support a causal association between GLP-1 medications and depression, suicidal ideation, or suicide. Of Yellow Card reports for GLP-1 medications between 2020 and 2025, the majority were non-serious and approximately 56% were gastrointestinal.

The supervised prescribing safety record

The main safety distinction is licensed medication versus unregulated products. Clinical trial evidence applies to medicines supplied by regulated pharmacies, not products bought from social media, beauty salons or unverified websites.

The MHRA has warned about counterfeit weight loss pens in the UK. Some fake pens have contained insulin instead of semaglutide, causing hospitalisation, hypoglycaemic shock and coma. Illegal products may also contain the wrong dose, bacteria, unknown ingredients or no active ingredient.

A legitimate UK weight loss injection should be prescribed by a qualified clinician, supplied by a GPhC-registered pharmacy and carry a UK marketing authorisation number such as PL, PLGB or PLNI.

96% relative reduction in prescribing errors compared to external benchmarks (0.17% vs 4.43%), with a 0.16% overall safety-incident rate. Published in Diabetes Obesity and Metabolism, 2025.

Voy outcome data. Published in Diabetes Obesity and Metabolism, 2025 (N=125,000+).

Who Are Weight Loss Injections Suitable For?

In the UK, Mounjaro is authorised for adults with a BMI of 30 or above, or 27 or above with at least one weight-related health condition. These criteria exist because treatment should be based on clinical need, not just weight-loss preference.

For eligible patients, Voy’s weight loss programme includes regulated prescribing, clinical monitoring, and access to licensed weight loss medication where clinically suitable.

The Bottom Line

Weight loss injections can be safe when prescribed correctly, monitored properly and obtained through regulated UK routes. The most common side effects are gastrointestinal and often improve over time.

Avoid unregulated sellers, never buy prescription injections without a prescription, and speak to a healthcare professional before starting treatment.