The Active Ingredients in Weight Loss Injections, Clearly Explained

The misconception worth correcting first: these aren't stimulants or slimming compounds

A surprisingly common assumption is that weight loss injections work the way older slimming medicines did, by speeding up metabolism, acting on the central nervous system, or suppressing appetite through stimulant-like mechanisms. That's not what tirzepatide or semaglutide do. Neither contains caffeine, amphetamine-related compounds, thyroid hormones, diuretics, or any of the ingredients historically found in black-market slimming products.

Both active ingredients are peptide molecules (short chains of amino acids) designed to bind to specific receptors in the gut, brain and pancreas. They mimic hormones your body already secretes in response to food. The result is a reduction in appetite, slower gastric emptying and better blood-sugar regulation. None of that requires a stimulant. It's a hormonal signal, not a chemical push.

Understanding this distinction matters practically. It means the side-effect profile is different from older medicines: the main things people notice are gastrointestinal (nausea, loose stools, indigestion) rather than raised heart rate or insomnia. It also means these medicines have no meaningful street value and are not controlled drugs, though counterfeit versions have still been seized in large numbers by the MHRA, which is its own serious problem covered below.

Tirzepatide: a dual-pathway molecule licensed in Mounjaro

Tirzepatide is a 39-amino-acid synthetic peptide that activates both the GIP (glucose-dependent insulinotropic polypeptide) receptor and the GLP-1 (glucagon-like peptide-1) receptor simultaneously. No other weight management medicine currently licensed in the UK works across both pathways. GIP receptors are found in fat tissue and the brain as well as the pancreas; GLP-1 receptors are concentrated in the gut and hypothalamus. Activating both together appears to produce a more pronounced reduction in food intake and body weight than GLP-1 activation alone, which is broadly what the SURMOUNT-1 trial, published in the New England Journal of Medicine, demonstrated, with average body-weight reductions around 20–21% at the highest dose over 72 weeks.

In the Mounjaro KwikPen, tirzepatide is dissolved in a buffered aqueous solution. The pen delivers a fixed volume subcutaneously once a week. For a full list of excipients (the inactive ingredients that stabilise the solution and control pH) the Mounjaro patient information leaflet and the Summary of Product Characteristics on the eMC are the right sources; these can matter if you have allergies to specific inactive compounds. If that's a concern for you, it's exactly the kind of thing to raise during a clinical consultation.

Mounjaro is available in six UK strengths, from 2.5 mg up to 15 mg, with treatment starting at the lower end to let your system adjust. You can read more about who these injections are licensed for and what a prescriber looks at during assessment.

Semaglutide: the GLP-1 analogue in Wegovy

Semaglutide is a 94% structurally identical analogue of human GLP-1, modified to resist the enzyme that would normally break it down within minutes. That modification (attaching a fatty-acid chain to the peptide backbone) allows it to bind to albumin in the bloodstream and remain active for roughly a week, which is why once-weekly dosing works. The active molecule itself is not new: semaglutide has been used in type 2 diabetes treatment (as Ozempic) for several years, but the 2.4 mg weekly dose in Wegovy is specific to the weight management licence and should not be conflated with diabetes dosing.

The NHS medicines page for semaglutide outlines the expected effects and side effects in plain language. The STEP 1 trial reported average body-weight reductions of around 15% over 68 weeks at 2.4 mg, with a higher-dose 7.2 mg formulation (approved by the MHRA in April 2026) showing results approaching tirzepatide in early data.

Wegovy's excipients include disodium phosphate dihydrate, sodium chloride, hydrochloric acid (for pH adjustment) and water for injections. Again, the patient information leaflet is the definitive source if you need the complete list. Semaglutide is now also available as a daily tablet (Wegovy tablets, approved June 2026), which uses a different excipient (SNAC, an absorption enhancer) to allow the peptide to cross the gut wall. You can read more about how a weight loss injection works and how it differs from the tablet form, since the two share the same active ingredient but are absorbed via completely different mechanisms.

What this means when you're assessing a product you've seen advertised

The ingredients question becomes especially important in the context of the unregulated market. The MHRA has warned repeatedly that pens sold via social media or websites offering these medicines without a prescription may contain unknown substances (in some cases, insulin) rather than the labelled active ingredient. A pen that looks identical to a genuine Mounjaro or Wegovy pen can contain something quite different. Large quantities of counterfeit tirzepatide pens have been seized across the UK, including from a manufacturing site raided in late 2025.

This is why the legal route matters beyond paperwork. A GPhC-registered pharmacy sources medicines through the licensed UK supply chain, where authenticity is verifiable at every stage. You can check any online pharmacy on the GPhC register before sharing any personal or medical information. Suspect products (or adverse reactions from anything you've taken) can be reported via the MHRA's Yellow Card scheme.

At nume, every medicine dispensed comes through that regulated supply chain, and every prescription is written by a GPhC-registered Independent Prescriber who has personally reviewed your consultation, not software, not a template. If you have questions about specific excipients or interactions relevant to your circumstances (for example, if you're wondering about thyroid conditions or cardiovascular history), those are exactly the conversations the clinical team is there for. You can also find general information on our FAQs page. When you're ready to find out whether treatment is clinically suitable for you, start your free consultation and a prescriber will review your answers the same day, covering all the weight loss injections we offer so you can be matched to whichever is right for your circumstances.