Weight loss drugs show results but raise safety questions

Three major reviews commissioned by the World Health Organization have confirmed what millions of people already know: weight loss drugs based on GLP-1 technology work. Medications like semaglutide, sold under brand names Ozempic and Wegovy, tirzepatide, marketed as Mounjaro and Zepbound, and liraglutide, known as Victoza and Saxenda, all produce substantial weight loss in people with obesity. But the same reviews that celebrate these results also raise a significant concern: most of the evidence supporting these drugs comes from studies funded by the companies that make them, and critical questions about long-term safety remain unanswered.

Background

GLP-1 drugs were originally developed to treat type 2 diabetes. They work by mimicking a natural hormone that slows digestion and makes people feel fuller for longer. In recent years, researchers have tested these medications in people with obesity, and the results have been striking enough to transform the weight loss market. The drugs have become so popular that an estimated 3.3 million UK adults are expected to use weight loss injections in 2026 alone.

The World Health Organization decided to examine the evidence around these medications to help shape global guidelines for their use. The organization commissioned three separate Cochrane reviews, which are considered among the most rigorous and independent assessments of medical evidence available. These reviews looked at the same three drugs that have dominated the weight loss conversation: tirzepatide, semaglutide, and liraglutide.

Key Details

How much weight do people actually lose?

The results are genuinely impressive. Tirzepatide produced the most dramatic weight loss, with people losing roughly 20 percent of their starting body weight over one to two years compared with those taking a placebo. Semaglutide came in close behind, also delivering around 20 percent weight loss. Even liraglutide, which showed more modest results, helped people lose about 4 to 5 percent of their body weight on average.

These numbers matter because they represent real changes in people's lives. Someone weighing 100 kilograms losing 20 kilograms is a significant transformation. The research also found that these benefits appear to continue as long as people stay on the medication.

The problem with the evidence

Here is where the reviews raise a red flag. Researchers found that many of the studies proving these drugs work were funded by the pharmaceutical companies that manufacture them. This creates what scientists call a conflict of interest. When a company funds research on its own product, there is always a question about whether the results might be skewed in the company's favor, even unintentionally.

Beyond the funding question, the reviews identified significant gaps in the evidence. Researchers found little or no difference between the GLP-1 drugs and placebo when it came to major cardiovascular events, quality of life, or death rates. This is important because many people taking these medications hope they will protect their hearts and extend their lives, not just help them lose weight.

Side effects and safety concerns

The medications do cause side effects, and they are common. Nausea and digestive problems are the most frequent complaints. Some people stopped taking the medication because these side effects were too difficult to tolerate. The reviews also highlighted that long-term safety data simply do not exist yet. Most studies tracked people for one to two years. What happens after five years? Ten years? Nobody knows.

The reviews conclude that longer-term, independently funded studies are important for guiding both medical practice and public health policy.

This gap in knowledge matters because doctors and patients are making decisions about whether to use these medications based on incomplete information. If someone takes a GLP-1 drug for five years and then stops, will they regain all the weight? Will they experience unexpected health problems? These questions cannot be answered with the current evidence.

What This Means

The WHO reviews essentially say this: these drugs work for weight loss, and that is real. But we do not yet understand the full picture of how safe they are long-term, and we should not trust studies that are mostly funded by the companies selling the drugs.

For people considering these medications, the message is mixed. The weight loss results are genuine and substantial. But anyone thinking about starting a GLP-1 drug should understand that they are based partly on evidence with potential conflicts of interest, and that long-term safety data do not yet exist.

For the pharmaceutical industry, the reviews suggest that more independent research is needed. Companies will need to fund studies conducted by researchers with no financial stake in the outcome, or allow independent researchers to conduct trials without company involvement. Without this shift, public trust in these medications will likely remain fragile.

The WHO has already taken one significant step. In December 2025, the organization issued its first global guideline recommending that GLP-1 drugs be used for long-term obesity treatment in adults. This recognition of obesity as a chronic disease requiring medication is a watershed moment. But the organization stopped short of including GLP-1 drugs on its Essential Medicines List for obesity treatment, a decision that reflects lingering uncertainty about long-term benefits and risks.

As these weight loss drugs continue to reshape the treatment landscape for obesity, the message from the WHO is clear: the drugs work, but we need better evidence before we fully understand what role they should play in global health.