Most weight loss injections contain a synthetic peptide that mimics hormones your gut naturally releases after eating. The active ingredient in the most widely prescribed options, like Wegovy and Zepbound, is a lab-engineered version of a hormone called GLP-1 that tells your brain you’re full. Beyond these prescription medications, you’ll also find clinics offering lipotropic shots made from vitamins and amino acids, though these are a very different category with far less evidence behind them.
GLP-1 Medications: The Core Ingredient
The most common weight loss injections on the market right now are GLP-1 receptor agonists. Your body naturally produces a hormone called GLP-1 (glucagon-like peptide-1) in the gut after meals. It signals your brain to reduce appetite, slows down how fast food leaves your stomach, and helps regulate blood sugar. The problem is that natural GLP-1 breaks down in minutes. The active ingredients in these injections are modified versions of that hormone, engineered to last days instead of minutes.
Semaglutide, the active ingredient in both Wegovy and Ozempic, shares 94% of its structure with natural human GLP-1. Three targeted modifications make it last roughly a week in your body instead of a few minutes. One change swaps out an amino acid to make the molecule resistant to the enzyme that normally chews up GLP-1 within minutes. Another attaches a fatty acid chain that lets the drug hitch a ride on albumin, a protein in your blood, which slows its clearance dramatically. The result is a half-life of about 165 hours, making a once-weekly injection possible.
Liraglutide, sold as Saxenda, uses the same basic approach but with a shorter-acting design. Each milliliter of Saxenda contains 6 mg of liraglutide along with inactive ingredients: a phosphate buffer to maintain stability, propylene glycol, phenol as a preservative, and water. Because liraglutide doesn’t last as long in the body as semaglutide, it requires daily injections rather than weekly ones.
Dual and Triple Hormone Agonists
Tirzepatide, the active ingredient in Zepbound, goes a step further by targeting two gut hormones instead of one. It activates both the GLP-1 receptor and the GIP receptor (glucose-dependent insulinotropic polypeptide). GIP is another hormone released after eating that plays a role in insulin response and fat metabolism. Tirzepatide’s structure closely mirrors native GIP in how it activates the GIP receptor, but it interacts with the GLP-1 receptor in an unusual way that causes less receptor burnout over time. This may be one reason it has shown slightly greater weight loss in clinical trials compared to semaglutide alone. Like semaglutide, tirzepatide has a fatty acid chain (a C20 fatty diacid) attached to extend its life in the body, enabling weekly dosing.
The inactive ingredients in Zepbound are minimal: sodium chloride, sodium phosphate as a buffer, and water for injection. Small amounts of hydrochloric acid or sodium hydroxide may be added during manufacturing to fine-tune the pH for stability.
A newer compound called retatrutide, still in clinical trials, targets three receptors: GLP-1, GIP, and the glucagon receptor. Adding glucagon receptor activity may increase the body’s energy expenditure on top of the appetite-suppressing effects of the other two pathways. Like the other injectable peptides, retatrutide uses a C20 fatty diacid to bind albumin and extend its duration.
How These Peptides Actually Work in Your Body
All of these medications do the same basic thing through slightly different pathways. They slow gastric emptying, meaning food sits in your stomach longer and you feel full sooner and for longer after meals. They reduce the release of glucagon, a hormone that raises blood sugar, after you eat. And they act on appetite centers in the brain to lower hunger and reduce food cravings. The combined effect is that people simply eat less without the same level of willpower required by dieting alone.
The appetite suppression is not subtle. Most people on these medications describe a fundamental shift in their relationship with food. Portions that previously felt normal start to feel excessive. Cravings for high-calorie foods often diminish significantly. This is the peptide working on receptors in the brain, not just in the gut.
Compounded Semaglutide: A Different Formulation
Some clinics and compounding pharmacies offer their own versions of semaglutide, often at lower prices than brand-name Wegovy. These compounded products may use salt forms of the molecule, such as semaglutide sodium or semaglutide acetate, rather than the base form used in the FDA-approved drug. The FDA has raised concerns about this distinction, noting that these salt forms are technically different active ingredients and that the agency has no data confirming they behave the same way in the body. Potency, absorption, and side effects could all differ. If you’re considering a compounded version, this is a meaningful distinction to understand.
Lipotropic Injections: Vitamins and Amino Acids
A completely separate category of weight loss injections exists at many med spas and wellness clinics. These are lipotropic shots, and their ingredients are nothing like the prescription peptides described above. The most common ingredient is vitamin B12, typically in the form of cyanocobalamin (synthetic, more stable, cheaper) or methylcobalamin (naturally occurring, possibly better retained by the body). Studies show the body absorbs about 49% of a dose of cyanocobalamin versus 44% of methylcobalamin, but methylcobalamin may be excreted less through urine, suggesting your body holds onto more of it.
Beyond B12, lipotropic shots often include some combination of methionine (an amino acid involved in fat metabolism), inositol (a sugar alcohol that plays a role in insulin signaling), and choline (a nutrient that helps transport fat out of the liver). You might also see L-carnitine, betaine, magnesium, niacin, pantothenate, or folate on the ingredient list. These combinations are sometimes called “MIC injections” after methionine, inositol, and choline.
The critical difference is evidence. The ingredients in lipotropic shots have shown some lipotropic (fat-metabolizing) effects in rodent studies, but dosages are not standardized by the FDA, and research in humans is limited. These are closer to vitamin supplements delivered by needle than to the powerful hormonal medications in the GLP-1 class. They are not FDA-approved for weight loss.
Setmelanotide: For Rare Genetic Obesity
One other injectable weight loss medication worth knowing about is setmelanotide, sold as Imcivree. It works through an entirely different mechanism, activating melanocortin-4 receptors in the brain that regulate hunger and energy balance. This medication is not prescribed for general obesity. It’s specifically approved for people aged six and older who have obesity caused by certain rare genetic disorders that disrupt these receptor pathways. For the small number of people with these conditions, it addresses the root cause of their weight gain in a way that GLP-1 drugs cannot.
What Keeps the Injection Stable
Peptides are fragile molecules. Without careful formulation, they degrade quickly, losing potency or breaking down into compounds that could cause injection site reactions. Every weight loss injection pen contains buffering agents, typically phosphate or glycine-based, that maintain a specific pH where the peptide is most stable and soluble. Preservatives like phenol prevent bacterial growth in multi-dose pens. Sodium chloride adjusts the solution’s tonicity so it matches your body’s fluids and doesn’t sting on injection. These inactive ingredients aren’t pharmacologically active, but they’re essential to keeping the drug effective from the manufacturing floor to the moment it enters your body.