For most people, FDA-approved weight loss injections like Wegovy and Zepbound have a well-established safety profile, but they come with predictable side effects and a short list of serious risks worth understanding. These medications have been studied in trials involving over 100,000 patients, and the data paints a nuanced picture: generally safe, not risk-free, and significantly safer than some alternatives being sold online.
How These Medications Work
The weight loss shots currently on the market mimic gut hormones that regulate appetite. When you eat, your intestines naturally release these hormones to signal fullness. The injections amplify that signal, reducing hunger and slowing digestion so you feel satisfied with less food. Wegovy (approved in 2021) targets one of these hormones, while Zepbound (approved in November 2023) targets two, which is partly why it tends to produce slightly more weight loss in head-to-head comparisons.
Both are approved for adults with a BMI of 30 or higher, or a BMI of 27 or higher with at least one weight-related condition like high blood pressure, type 2 diabetes, or high cholesterol. They’re meant to be used alongside diet and exercise, not as a standalone fix.
The Side Effects Most People Experience
Nausea, vomiting, diarrhea, and constipation are by far the most common complaints. Roughly 50% to 60% of patients experience some combination of these gastrointestinal symptoms, especially in the first few weeks. The medications are prescribed at low starting doses and gradually increased for exactly this reason. For most people, these symptoms fade as the body adjusts, though some never fully shake the nausea.
These side effects are dose-dependent, meaning they tend to get worse each time your dose increases. If you’re on a titration schedule and a new dose hits you hard, your prescriber can hold you at the previous dose longer before stepping up again.
Serious Risks to Know About
The more concerning risks are uncommon but real. Every approved GLP-1 weight loss medication carries a boxed warning about thyroid cancer, specifically a rare type called medullary thyroid carcinoma. This warning comes from animal studies where rodents developed thyroid tumors. In humans, a large Scandinavian study tracking over 145,000 patients on these drugs found thyroid cancer rates of about 1.3 per 10,000 patient-years, which was actually slightly lower than the comparison group on a different class of diabetes medication. The risk appears to be very small, but these drugs are completely off-limits if you or a close family member has a history of medullary thyroid cancer or a genetic condition called MEN2 syndrome.
Gastroparesis, a condition where the stomach empties too slowly, is another risk that’s gotten attention. Because these drugs work partly by slowing digestion, they can tip some people into clinically significant stomach paralysis. Research from Cleveland Clinic found that patients on GLP-1 medications had increased odds of a gastroparesis diagnosis at every time point from six months through two years of use. Symptoms include persistent nausea, bloating, and feeling full long after eating. If you already have slow gastric emptying, this is a conversation to have before starting.
Pancreatitis and gallbladder problems have also been reported, though large-scale incidence data is still being collected. Rapid weight loss itself increases gallstone risk regardless of how you lose the weight, so it’s hard to separate the drug’s effect from the effect of losing a significant amount of weight quickly.
Muscle Loss Is a Real Tradeoff
One safety concern that doesn’t get enough attention is what happens to your body composition. Studies suggest that 25% to 39% of the total weight lost on these medications comes from lean mass, primarily muscle, rather than fat. Over 36 to 72 weeks of treatment, that can add up to a meaningful amount of muscle.
This matters most for older adults, who are already losing muscle with age and can ill afford to accelerate the process. Losing muscle reduces your metabolic rate, weakens your bones, and increases fall risk. Resistance training while on the medication is the most effective way to counteract this, and some clinicians now consider it a non-negotiable part of the treatment plan.
Cardiovascular Benefits Over the Long Term
The safety picture isn’t all caution. The SELECT trial, one of the largest and longest studies of semaglutide in people with obesity, found that the drug reduced major cardiovascular events (heart attacks, strokes, and cardiovascular death) by 20% compared to placebo in patients who already had heart disease. This was in people without diabetes, meaning the benefit came from something beyond blood sugar control. Serious adverse events were actually less common in the treatment group than in the placebo group, though more patients on the drug discontinued due to gastrointestinal side effects.
This finding was significant enough that Wegovy earned a separate FDA indication for reducing cardiovascular risk in people with established heart disease and obesity.
The Suicidal Thoughts Question Is Settled
Early reports raised concern about a possible link between GLP-1 medications and suicidal thoughts. The FDA conducted a comprehensive review, analyzing 91 placebo-controlled trials that included nearly 108,000 patients. The results showed no increased risk of suicidal ideation, suicidal behavior, anxiety, depression, irritability, or psychosis. The FDA concluded there is no causal relationship between these drugs and suicidal thoughts, and in 2025 it formally requested that manufacturers remove the suicidal behavior warning from their labels.
Compounded Versions Are a Different Story
If you’re considering a cheaper, compounded version of semaglutide or tirzepatide from an online clinic or compounding pharmacy, the safety calculus changes significantly. The FDA has flagged several specific concerns. Some compounders use salt forms of semaglutide (like semaglutide sodium or semaglutide acetate) that are chemically different from the active ingredient in approved drugs. The FDA has no data confirming these salts behave the same way in the body.
As of July 2025, the FDA had received 605 adverse event reports tied to compounded semaglutide and 545 tied to compounded tirzepatide. Those numbers are likely undercounts because state-licensed pharmacies aren’t required to report adverse events the way drug manufacturers are. Multiple reports involved hospitalization from dosing errors, where patients or even healthcare professionals miscalculated the dose. Unlike the brand-name pens, which deliver a pre-measured dose, compounded versions often require you to draw up the correct amount from a vial, creating room for mistakes.
Some compounded products have also been prescribed at doses beyond what’s in the FDA-approved labeling, with faster titration schedules or higher single doses. Several of these cases resulted in serious gastrointestinal symptoms that required medical attention. The cost savings of compounded versions come with real uncertainty about purity, dosing accuracy, and chemical equivalence.
Who Should Not Take These Medications
Beyond the thyroid cancer contraindication, these drugs aren’t appropriate for everyone. They haven’t been adequately studied in pregnant or breastfeeding women, and manufacturers recommend stopping the medication at least two months before a planned pregnancy due to its long half-life. People with a history of severe gastrointestinal disease, including gastroparesis, should approach with caution. And because these medications slow stomach emptying, they can affect the absorption of other oral medications you take, which your prescriber needs to account for.
The bottom line is that FDA-approved weight loss injections have a favorable risk-benefit profile for people who meet the criteria, particularly those with obesity-related health conditions. The side effects are frequent but mostly manageable, the serious risks are uncommon, and the cardiovascular data is genuinely encouraging. The biggest safety red flags surround compounded and unregulated versions, where quality control and dosing accuracy can’t be guaranteed.