If you are using fat jabs such as Mounjaro or Wegovy, you might wonder where you should jab to best optimise your weight loss, particularly if you’ve hit a plateau or you are suffering from side-effects that are slowing your progress.

Social media message boards and TikTok videos are full of chatter suggesting different ways to inject GLP-1s to boost your weight loss. But what’s the truth behind these tricks? Experts reveal EXACTLY where you should inject for best results, and the areas you should NEVER jab.

What’s the best spot to avoid side-effects?

After cost, side-effects such as nausea, diarrhoea and vomiting are the main reasons people stop treatment, something which almost always leads to weight regain. It can also mean slowing down or even halting your weight loss if side-effects mean you can’t tolerate a higher dose.

So, could changing where you inject help? Weight-loss drug expert Dr Michael Crotty, clinical lead for obesity at the Irish College of GPs, says: ‘Some people report anecdotally that gastrointestinal side-effects improve when they switch sites.’

However, he adds that this is more likely to be due to the body naturally adjusting to the medication rather than a true effect of injection location.

Mandeep Kaur, pharmacist and Weight Loss Service lead at Boots Online Doctor, says: ‘The medication should work the same way regardless of your injection site, there should be no clinical difference in the side effects caused by injecting into different areas’.

However, a 2015 study on an experimental GLP-1 (not Wegovy or Mounjaro) found faster absorption with arm or thigh injections, but also increased side-effects, leading researchers to suggest the abdomen as the best site. Anecdotally, the abdomen is also seen as the least painful area as it generally has more fat than the thigh or arm.

Where should you NEVER inject?

Dr Crotty says: ‘These medicines are specifically designed to be given subcutaneously – into the fatty tissue just under the skin – not into muscle or a vein.’

Kaur adds: ‘This ensures gradual absorption into the bloodstream, which is essential for maintaining effective medication levels over a longer period and minimising any side-effects.’

This is why you should never inject into naturally lean, muscled areas, such as your shoulders or lower legs. ‘Injecting into muscle could alter how the drug is absorbed and may increase discomfort. The aim is to ensure consistent, predictable absorption,’ says Dr Crotty.

Where else shouldn’t you inject?

Mandeep Kaur says: ‘It is essential to avoid injecting into areas with scars, moles, bruises, or skin abnormalities, as these can affect absorption and increase the risk of complications.’

The one reason why switching sites could make your jabs more effective

Dr Crotty says, ‘Rotating injection sites is important because repeatedly using the exact same spot can lead to local tissue changes such as thickening, scarring, or lipohypertrophy – a knotty lump of fatty tissue under the skin’.

He says that continuing to inject into this damaged skin ‘can interfere with how the medication is absorbed over time’. This doesn’t mean you have to change to a completely different body area every week. Dr Crotty says: ‘A simple approach is to move at least a couple of centimetres from the previous site, and when using the abdomen, to stay at least five centimetres away from the navel.’

This is because the area around the belly button has less fat and more dense tissue. Injecting here can lead to uneven, slower, or unpredictable drug absorption.

Can a simple switch supercharge your weight loss?

If your weight loss has stalled, you might be tempted by claims that injecting into different body areas will speed up the process.

The official guidelines issued with GLP-1 medications state that you should inject into one of three fatty areas of the body, your abdomen, upper or outer thighs or the back of the upper arm – the tricep area. Some online sources claim that switching sites – such as from your abdomen to your arm – can increase the dose that hits your bloodstream.

Dr Crotty says: ‘There is no convincing scientific evidence that choosing different approved injection sites has any clinically meaningful impact on effectiveness or side effects with semaglutide (Wegovy) or tirzepatide (Mounjaro).’

A study by the Mounjaro manufacturer, Lilly, backs him up, finding that blood levels of the medication were ‘similar’ regardless of which of the three sites people opted for.

Can you spot-reduce by injecting into your fattiest areas?

Some GLP-1 influencers claim that you can spot-reduce (target a certain area of the body for fat reduction) by jabbing that part. There’s no scientific evidence for this – though thicker layers of fat can make the injections more comfortable.

However, an intriguing US study is using GLP-1 drugs to treat people affected by the disease lymphoedema, which can occur as a complication of obesity or cancer treatment.

The condition causes lymph fluid, part of the immune system, to accumulate, usually in the arms or legs, causing progressive swelling and skin thickening. The condition can also eventually lead to stubborn fat build-up in these areas.

The first US study to examine whether weight-loss jabs can help prevent and treat lymphoedema is under way. It is led by Dr Joseph Dayan, head of the Institute for Lymphatic Surgery and Innovation in New Jersey, US.

And last year, Russian scientists published a paper in the journal Advanced Pharmaceutical Bulletin suggesting that the ‘localised administration’ of GLP-1 drugs could ‘suppress inflammatory pathways’, restore damaged lymph function and may shrink affected limbs.

However, Dr Crotty says this definitely isn’t one to try at home. ‘I would not advise injecting into areas affected by lymphoedema,’ he says. Standard clinical advice is to avoid injections in these regions where possible, due to concerns around infection risk and potentially altered absorption.