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← All articlesWeight loss injections are once-weekly medications from the GLP-1 family, prescribed after an evaluation and taken under a provider's supervision for months, not weeks. In the two largest trials, people lost an average of 15 to 21 percent of their body weight over about a year and a half, and most of the side effects were stomach-related and faded as the dose settled. The injection itself is the easy part. What separates a result that lasts from one that stalls or reverses is everything around it: the screening, the dose schedule, the lab work, the muscle you keep, and the plan for month seven. This guide explains how a supervised program in Tarzana handles each of those, so you know what to ask before you start anywhere.
The medications people mean by "weight loss injections" are GLP-1 receptor agonists. Semaglutide acts on one gut-hormone pathway. Tirzepatide acts on two, GLP-1 and GIP. Both are given as a small subcutaneous injection once a week, both are escalated slowly from a low starting dose, and both work the same way at the level that matters to a patient: they slow stomach emptying, blunt appetite signals in the brain, and make it possible to eat less without the constant hunger that defeats most diets.
They are prescription drugs. In the United States the FDA has approved specific branded versions for chronic weight management in adults with obesity, or with overweight plus a weight-related condition. Compounded versions made by licensed pharmacies exist as well, and the FDA has been clear about how to think about those, which is covered below.
What they are not: a shortcut that works on its own. The trials that produced the headline numbers all paired the medication with a lifestyle program, and the people who kept their results were the ones who used the quieter appetite to change how they ate and moved.
Two studies define the evidence base. Both were published in the New England Journal of Medicine, both were placebo-controlled, and both ran for well over a year.
1,961 adults with overweight or obesity, no diabetes, took once-weekly semaglutide at the full 2.4 mg dose or a placebo for 68 weeks, alongside lifestyle counseling. The semaglutide group lost an average of 14.9 percent of body weight. The placebo group lost 2.4 percent. More than 86 percent of the people on the medication lost at least 5 percent, against 31.5 percent on placebo. The most common side effects were nausea and diarrhea, described as typically transient and mild to moderate. About 4.5 percent stopped because of stomach side effects.
2,539 adults with obesity, no diabetes, took once-weekly tirzepatide at 5, 10 or 15 mg or a placebo for 72 weeks, including a 20-week dose-escalation period. Average weight change at week 72 was 15.0 percent at 5 mg, 19.5 percent at 10 mg and 20.9 percent at 15 mg, against 3.1 percent on placebo. Between 85 and 91 percent of people on the medication lost at least 5 percent. Side effects were again mostly gastrointestinal, mostly mild to moderate, and clustered during the escalation weeks. Discontinuation for side effects ran 4 to 7 percent depending on dose.
Three things to take from those numbers. First, the averages hide a wide spread: some people lose far more, some respond modestly, and nobody can tell you in advance which you will be. Second, the results took 68 to 72 weeks. Anyone promising a transformation in eight weeks is selling something the evidence does not support. Third, the escalation period is where most of the discomfort lives, which is exactly why how your dose is managed matters more than which molecule you start on. Our comparison of tirzepatide and semaglutide goes deeper on the head-to-head question.
The approved indication is adults with a body mass index of 30 or above, or 27 or above with at least one weight-related condition such as high blood pressure, high cholesterol, sleep apnea or prediabetes. That is the starting line, not the finish. A responsible evaluation also looks at:
At Contour Medical Clinic the evaluation is a provider visit plus a baseline lab panel drawn at a Quest location near you, reviewed before anything is prescribed. Our provider decides whether medication fits, which one, and at what pace. If the answer is no, the visit still produced a clear picture of where your metabolism stands and what to do instead. The two-minute pre-screen is a reasonable first step if you are not sure you qualify.
This is the part most people never see described, and it is where programs differ.
Both medications begin at a low dose that does very little for weight and a lot for tolerance. The point of month one is to let the gut adapt. Expect appetite to drop noticeably within the first two weeks, some early fullness, and possibly mild nausea in the day or two after each injection. A check-in at the end of the month confirms you are tolerating it before the dose moves.
The FDA labels for both medications step the dose up roughly every four weeks until the maintenance dose is reached. A supervised program does not escalate on a calendar; it escalates when the previous step is comfortable. Moving faster than the label is one of the specific errors the FDA has flagged in its adverse-event reports, and it is the most common reason people quit in month two. Our guide to what is normal and what is not during escalation covers the symptoms worth a call.
The scale cannot tell you whether the weight you are losing is fat or muscle. A body composition scan can, and in a well-run program it is repeated on a schedule. Losing lean mass along with fat is a known effect of rapid weight loss on these medications, and it is preventable with adequate protein and resistance training. We wrote about how to protect muscle during GLP-1 weight loss because it is the single most under-discussed part of the whole process.
Weight loss on these medications slows and eventually plateaus. That is physiology, not failure, and a plateau has a short list of causes a provider can work through. The harder conversation is what happens at the end. The trial data are consistent: people who stop the medication without a maintenance plan regain a large share of the weight within a year. A supervised program talks about the exit from the first visit, which is the subject of our piece on stopping a GLP-1 without regaining the weight.
Because the branded medications are expensive and were in shortage for a long stretch, many programs, including ours, use versions prepared by licensed compounding pharmacies. The FDA updated its guidance on this in October 2026, and its position is worth reading in plain terms rather than through marketing.
None of that is a reason to avoid compounded medication. It is a reason to get it through a clinic that names its pharmacy, does the screening, manages the dose and answers the phone. Our earlier guide on how to choose a GLP-1 provider near you turns these points into questions to ask.
Averaging the trial data across a year: a person starting at 220 pounds who responds like the typical participant might expect to be somewhere between 185 and 190 pounds at the one-year mark on tirzepatide, and a little higher on semaglutide, with most of the loss landing between months two and nine. Hunger is quieter from week two. Clothes change before the scale does, because fat loss around the waist shows up early. Energy often dips in the escalation months and returns once the dose stabilizes and protein intake catches up.
Two honest caveats. Individual results vary widely, and the trials measured outcomes in people who also changed what they ate and how they moved. A supervised program in the San Fernando Valley is not selling a number. It is selling the structure that makes a number reachable and, more importantly, keepable.
The needle is very short and thin, similar to an insulin pen, and goes into the fat of the abdomen, thigh or upper arm. Most people describe it as a brief pinch. The first one is done in the clinic so you are shown how.
Appetite changes within the first two weeks. Meaningful weight change usually begins in month two as the dose increases, and continues for most of the first year. The trials ran 68 to 72 weeks to reach their reported averages.
Without a maintenance plan, most people regain a large share within a year. With a plan built around protein, strength training and a gradual taper decided with the provider, regain can be limited. This is discussed from the first visit, not the last.
Not legitimately. These are prescription medications. The FDA names "no screening by a licensed doctor" as a warning sign of an illegal or unsafe seller.
Tirzepatide produced larger average losses in its trial, but the two trials were not head-to-head, and tolerance, history and cost all factor in. Our provider makes that call with you after your evaluation and labs.
Weight loss injections work, and the evidence behind them is as strong as anything in metabolic medicine right now. They work best, and most safely, inside a program that screens properly, escalates at your pace, tracks body composition rather than just weight, uses a named licensed pharmacy, and plans the ending from the beginning. That is what a supervised program means, and it is the difference between a year of results and a year of regret.
If you are in Tarzana, Encino, Woodland Hills or Sherman Oaks and want to find out whether you qualify, see our GLP-1 weight loss program in Tarzana page, where you can book a free consultation on the calendar and read what the program includes. Our provider decides what fits after your evaluation.
Sources: Wilding et al., "Once-Weekly Semaglutide in Adults with Overweight or Obesity" (STEP 1), N Engl J Med 2021;384:989-1002. Jastreboff et al., "Tirzepatide Once Weekly for the Treatment of Obesity" (SURMOUNT-1), N Engl J Med 2022;387:205-216. U.S. Food and Drug Administration, "FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss," content current as of October 1, 2026.
Written by: Contour Medical Clinic Editorial Team