We’re still figuring out the side effects of GLP-1 weight-loss drugs
This week my colleague Antonio Regalado had an interesting update on GLP-1 weight-loss drugs. According to research presented at an aging meeting in Boston, these drugs seem to affect at least some measures of biological age.
GLP-1 drugs are medications designed to help with weight loss by reducing hunger. They belong to a class called GLP-1 receptor agonists, which mimic a natural hormone in the body called glucagon-like peptide-1 (GLP-1). This hormone regulates appetite and blood sugar levels. These drugs were initially approved to treat diabetes and obesity but are now also prescribed for chronic kidney disease and cardiovascular risks. In 2026, 11% of Americans reported taking a GLP-1 drug for weight loss, up from 3% in 2024, according to a Gallup poll. The drugs are produced by companies like Eli Lilly and Novo Nordisk, which dominate the market. Beyond weight loss, research suggests potential benefits for substance-use disorders, neurodegenerative diseases, and even cancer, though these uses are still under investigation.
Recent research presented at an aging meeting in Boston in 2026 suggests that GLP-1 drugs may slow biological aging. Studies by Eli Lilly and Novo Nordisk found that overweight and diabetic individuals taking these drugs had a biological age approximately two to three years younger than similar individuals not taking the drugs. Biological age refers to how old a person's body appears based on cellular and molecular markers, rather than their actual chronological age. This finding aligns with growing interest in GLP-1 drugs as potential longevity drugs, though their effects are currently limited to people who are overweight or diabetic. Further research is needed to confirm these results and understand their long-term implications.
The most frequently reported side effects of GLP-1 drugs are gastrointestinal issues, including nausea, vomiting, diarrhea, and constipation. These symptoms occur because the drugs slow digestion to reduce hunger. Less common but more serious side effects include pancreatitis, an inflammation of the pancreas, and gallstone disease, which involves the formation of hardened deposits in the gallbladder. These risks are highlighted in drug safety updates from authorities like the UK's Medicines and Healthcare products Regulatory Agency. While these side effects are well-documented, their frequency and severity vary among individuals, and not everyone experiences them.
There are concerns that GLP-1 drugs might, in rare cases, cause damage to the nerves behind the eye, a condition known as non-arteritic anterior ischemic optic neuropathy (NAION). This condition can lead to sudden vision loss. Dozens of lawsuits have been filed against Novo Nordisk and Eli Lilly by individuals claiming their vision problems were caused by these drugs. Both companies deny the allegations, and research on the link remains inconclusive. The US Food and Drug Administration (FDA) continues to monitor this potential side effect as part of its ongoing evaluation of GLP-1 drugs.
Emerging evidence suggests a possible link between GLP-1 drugs and hair loss or nail disorders. A study by researchers at George Washington University School of Medicine and Health Sciences found that GLP-1 use was associated with an increased risk of hair loss, though the condition is not thought to be common. Another smaller study presented at a conference in Austria in 2026 reported that 66% of GLP-1 users experienced at least one nail disorder, such as nail detachment. However, nail conditions were also common among non-users, so the direct causal relationship remains unclear. These findings highlight the need for further research to understand the full scope of these side effects.
The use of GLP-1 drugs in children has surged in recent years, with a 310-fold increase in prescriptions for eight- to 11-year-olds with obesity in the US between 2019 and 2026. Despite this rise, pediatric side effects remain poorly understood due to limited data. In October 2026, the World Health Organization (WHO) released its first global guidelines on managing obesity in children and adolescents, explicitly recommending against pharmacological treatment for kids under 10. The WHO cited a lack of evidence regarding both the benefits and potential adverse effects of these drugs on growth, development, and mental health. The guidelines emphasize the need for caution and further research before widespread use in young children.
Regulatory bodies and researchers continue to investigate the safety and efficacy of GLP-1 drugs as their popularity grows. The FDA and other agencies are actively monitoring reports of side effects, including vision issues and gastrointestinal problems. Meanwhile, hundreds of studies are underway to explore additional benefits and risks. For example, research is examining whether these drugs could help with neurodegenerative diseases or cancer, though results are preliminary. As of October 2026, the scientific community is still in the process of fully understanding the long-term implications of GLP-1 drugs, and new findings are expected to emerge as more data becomes available.
