Weight Loss Injections: The Parts Nobody Puts in the Advert

Let us start by saying the unfashionable thing: these medicines work. They are the most effective non-surgical weight loss treatment that has ever existed, and the scepticism they attract is often unearned.

What follows is not an argument against them. It is the set of things that tend not to make it into the marketing, and that people deserve to know before they start something they may be on for years.

What they actually do

Semaglutide and tirzepatide mimic gut hormones released after eating. They slow how quickly the stomach empties and act on appetite signalling in the brain.

The effect people describe is not willpower arriving. It is the constant background noise about food going quiet. Many are surprised by how much mental space that frees up — and that reaction is itself informative, because it suggests appetite regulation was never simply a matter of trying harder.

They do not burn fat, raise metabolism, or target any particular part of the body. They reduce how much you eat.

What the trials showed

In the major trials, average weight loss over roughly 15 to 18 months was around 15% of body weight with semaglutide and around 20% with tirzepatide at the higher doses, alongside dietary support.

Two caveats on those headline numbers. They are averages with a very wide spread — some people lose far more, and a minority lose almost nothing. And they were achieved with structured dietary and activity support running alongside, which is not what most people buying online receive.

The part that gets left out

When you stop, most of the weight comes back.

This is not a footnote or a failure of discipline. In the withdrawal studies, participants regained roughly two thirds of what they had lost within about a year of stopping, and cardiometabolic improvements went with it.

The reason is straightforward. The medicine suppresses appetite while you take it. Stop, and appetite returns — to a body that now needs fewer calories than it did, because it is smaller.

The honest way to think about these drugs is as a long-term treatment for a long-term condition, in the same way as blood pressure medication. Nobody expects their blood pressure to stay down after stopping ramipril. Anyone framing a GLP-1 as a course you complete is either misinformed or selling something.

That has a financial consequence worth confronting up front: this is an ongoing cost, not a one-off. Starting a treatment you cannot afford to continue means paying to lose weight you will then regain.

You lose muscle as well as fat

A meaningful share of the weight lost on these drugs — as on any rapid weight loss — is lean tissue rather than fat. That matters for strength, for metabolic rate, and for how well you age.

It is also largely preventable, and the two things that prevent it are unglamorous:

• Eat enough protein. This is genuinely difficult on a drug that removes your appetite, which is exactly why it needs deliberate attention rather than being left to chance
• Resistance training, twice a week. Not cardio — resistance. This is the single most effective thing you can do to keep the weight you lose off the right tissue

Anyone prescribing these medicines without mentioning protein and strength work is not managing your treatment properly.

Side effects, honestly

Common: nausea, vomiting, constipation, diarrhoea, reflux, burping, fatigue. Most are worst after a dose increase and settle. Slow titration is the main defence, and there is rarely a good reason to rush it.

Less common but important: gallstones — rapid weight loss causes them regardless of the drug; pancreatitis, which is rare and presents as severe upper abdominal pain boring through to the back and needs urgent assessment; and marked dehydration if vomiting is not managed.

The facial and body changes that get discussed online are simply what losing 20% of your body weight looks like at any speed. They are not a specific drug effect.

Two safety points that are widely missed

Contraception

Tirzepatide can reduce the effectiveness of the oral contraceptive pill, because delayed stomach emptying affects absorption. If you take the pill, you need an additional barrier method — or a switch to a non-oral method — for four weeks after starting, and for four weeks after every dose increase.

This is not obscure; it is in the prescribing information. It is also routinely not mentioned by online services, and unplanned pregnancies have followed.

Relatedly: these medicines should be stopped well before trying to conceive, and they are not for use in pregnancy or breastfeeding. Fertility can also improve as weight falls, particularly in PCOS — which is welcome for some and a surprise for others.

Operations and procedures

Tell your surgeon and anaesthetist that you take one of these medicines, well in advance of any operation, endoscopy or procedure involving sedation. Because they slow stomach emptying, there can be food left in the stomach even after a standard fast, which carries a risk during anaesthesia. Guidance now generally involves pausing the medicine beforehand.

Who should not take them

• Anyone who is pregnant, breastfeeding, or trying to conceive
• A personal or family history of medullary thyroid cancer, or the MEN2 syndrome
• Previous pancreatitis
• Severe gastroparesis or significant gut motility disorders
• Anyone with a current or past eating disorder — not an automatic exclusion, but a situation that needs proper assessment rather than an online form, because appetite-suppressing drugs interact badly with restrictive illness

What good prescribing looks like

If you are considering starting, this is a reasonable checklist to hold a provider to:

• Baseline blood tests — HbA1c, liver and kidney function, thyroid function and lipids. Not because the drug requires them, but because this is the moment the underlying picture is worth understanding
• A real conversation about eligibility, rather than a self-declared height and weight
• A named clinician you can contact when the nausea starts, not a chatbot
• Slow, flexible dose titration — staying at a dose that is working is entirely legitimate
• Advice on protein and resistance training from the start
• An honest discussion about what happens when you stop, before you start

Be wary of any service that will not ask you anything. These are prescription-only medicines for good reasons, the unregulated market includes falsified pens, and "no consultation needed" is a warning rather than a convenience.

The bottom line

For someone whose weight is genuinely affecting their health, these drugs can be transformative, and being sniffy about them helps nobody.

But they are a long-term treatment, not a course. Most of the weight returns if you stop. Some of what you lose is muscle unless you actively protect it. And they need prescribing by someone who will still be there in three months.

If that sounds like a reasonable trade, they are a very good option. If nobody has said any of it to you before taking your card details, find a different provider.