Safety is the top concern for anyone considering weight loss medication — and rightly so. Here's what the clinical evidence and regulatory landscape actually tell us.
The regulatory picture
All GLP-1 weight loss injections available in the UK have been approved by the MHRA (Medicines and Healthcare products Regulatory Agency) following rigorous review of clinical trial data. They've also been approved by the FDA (US), EMA (Europe), and TGA (Australia). This means they've met strict standards for safety, efficacy, and quality.12
NICE (the National Institute for Health and Care Excellence) has independently evaluated the evidence and recommended both Mounjaro and Wegovy for use on the NHS — a process that includes detailed cost-effectiveness and safety analysis.3
The clinical trial evidence
Semaglutide and tirzepatide have been studied in tens of thousands of patients across multiple large-scale, randomised clinical trials. The STEP programme (semaglutide) included over 15,000 participants, and the SURMOUNT programme (tirzepatide) included over 7,000. The SELECT cardiovascular trial alone enrolled 17,604 people and followed them for nearly 3 years.4
These trials consistently show that the medications produce significant weight loss with a manageable side effect profile. Serious adverse events are uncommon, and the overall safety profile is considered acceptable relative to the health benefits of substantial weight loss.
Known risks
No medication is risk-free. The known risks of GLP-1 medications include gastrointestinal side effects (very common but usually temporary), pancreatitis (~1 in 100), gallbladder problems, and the Mounjaro-specific contraception interaction. For a comprehensive breakdown, see our side effects guide.
Long-term data is still being gathered — these medications have been in widespread use for weight loss for only a few years. Post-marketing surveillance by the MHRA and other regulators continues to monitor for any emerging safety signals.
The counterfeit risk
One genuine safety concern has nothing to do with the medications themselves. The MHRA has issued repeated warnings about counterfeit weight loss injection pens being sold through unregulated online sources and social media. Since January 2023, the MHRA has seized 369+ potentially counterfeit Ozempic pens and reported fake Saxenda pens circulating in the UK.5
Counterfeit pens may contain incorrect doses, wrong ingredients, or contaminants. To protect yourself:
- Only obtain medication from a GPhC-registered pharmacy
- Never buy from social media, unregulated websites, or individuals
- Check that your prescriber is registered with the GMC, GPhC, or NMC
- Verify the pharmacy at pharmacyregulation.org
- Report suspected counterfeit medicines to the MHRA Yellow Card scheme
The bottom line
When prescribed by a qualified healthcare professional and obtained from a legitimate pharmacy, GLP-1 weight loss injections are considered safe for the vast majority of eligible patients. They've been through extensive clinical testing, approved by multiple international regulators, and recommended by NICE for NHS use. Like all medications, they carry some risks — but for people with obesity, the health risks of remaining at a high weight generally outweigh the risks of treatment.
The key is to use them under appropriate medical supervision, obtain them from regulated sources, and report any concerns to your prescriber.