Weight loss injections — commonly called “fat jabs” — have become one of the most discussed topics in UK health and fitness circles. Medications such as semaglutide (Wegovy, Ozempic) and tirzepatide (Mounjaro) are now prescribed through NHS pathways and private clinics alike. As a certified personal trainer working closely with clients managing weight alongside medical teams, I’ve seen first-hand how these treatments can be transformative — but side effects are real, common, and worth understanding thoroughly before you begin.
How Fat Jabs Work and Why Side Effects Occur
GLP-1 receptor agonists mimic hormones that regulate appetite, gastric emptying, and blood sugar levels. Because these medications slow how quickly your stomach empties food into the small intestine, gastrointestinal side effects are the most frequently reported. The NHS semaglutide guidance confirms that most side effects are dose-dependent, meaning they often worsen during dose escalation periods and ease as your body adjusts.
Most Common Side Effects: What the Evidence Says
Clinical trial data and real-world evidence consistently highlight the following side effects across both semaglutide and tirzepatide treatments. The NICE technology appraisal TA875 for semaglutide documents the adverse event profile comprehensively:
- Nausea — affects up to 44% of users, particularly in the first 4–8 weeks
- Vomiting — reported in approximately 24% of patients during dose titration
- Diarrhoea — common during early treatment phases
- Constipation — paradoxically, some users experience the opposite
- Injection site reactions — redness, itching or mild swelling at the injection site
- Fatigue and headaches — often linked to reduced caloric intake
- Acid reflux (GERD) — due to delayed gastric emptying
Side Effect Comparison: Semaglutide vs Tirzepatide
| Side Effect | Semaglutide (Wegovy) | Tirzepatide (Mounjaro) |
|---|---|---|
| Nausea | Very common (≥1 in 10) | Very common (≥1 in 10) |
| Vomiting | Common | Common |
| Diarrhoea | Common | Common |
| Constipation | Common | Very common |
| Hypoglycaemia risk | Low (without insulin) | Low (without insulin) |
| Pancreatitis (rare) | Reported | Reported |
| Injection site reaction | Mild | Mild |
Sarah’s Clinical Experience: What Clients Report
From Sarah Mitchell, Level 3 PT: “Over the past two years, I’ve supported several clients who were prescribed weight loss injections through both NHS specialist pathways and private clinics. The most consistent pattern I’ve observed is that nausea peaks around weeks two to four of each new dose increment. Clients who managed their meal sizes carefully — eating smaller portions, avoiding high-fat foods immediately after injecting — reported significantly more manageable symptoms. One client described the first six weeks as genuinely difficult but found that consistent hydration and gentle movement, rather than high-intensity sessions, helped them feel more in control. None of my clients discontinued treatment solely due to side effects, though all required close monitoring from their prescribing clinician.”
Practical Management Strategies
The NHS healthy weight guidance emphasises that lifestyle support alongside medication dramatically improves outcomes. Here are evidence-informed management tips:
