Jennifer Lynn-Pullman had been taking Wegovy for a few months when she began shivering during a walk on a hot August day.

“It was about 85 degrees – I should have been sweating,” said Lynn-Pullman, a 48-year-old dietitian from Pennsylvania. A weeklong food log revealed she was eating less than 900 calories a day – well below the 2,000 daily calories recommended by the US Department of Agriculture for a moderately active woman in her 40s and below the 1,200-1,500 calories commonly advised for women on weight loss drugs.

Since starting the weekly injections, she’d been losing a pound or two a week – which she considered reasonable. Yet she’d fallen into a dangerous trap experts warn could become more common as GLP-1 weight loss drugs become more widely used: She wasn’t eating enough calories.

Lynn-Pullman, also a certified obesity and weight management specialist, upped her calorie intake and added more protein. Within a week, she felt better – no more chills. Because of her professional background, she was able to spot and address her symptoms quickly. But her experience raises crucial questions: If this can happen to a nutrition expert, what about someone without her expertise? Could they miss early symptoms, and would the health fallout be worse?

The class of obesity medications known as GLP-1 agonists – also approved to treat type 2 diabetes and to lower the risk of heart and blood vessel problems in some adults with heart disease – is highly effective for weight loss. The drugs, which include semaglutide (Ozempic, Rybelsus, Wegovy) and tirzepatide (Mounjaro, Zepbound), work by mimicking the body’s natural GLP-1 hormone. They act on brain receptors to slow digestion, increase feelings of fullness, and reduce appetite.

But doctors have growing concerns: Some patients on GLP-1s unintentionally eat too little and become malnourished. A new study in Obesity Reviews supports these worries, finding that many patients don’t get enough nutritional guidance, leaving them vulnerable to deficiencies.

“We see cases where people take a GLP-1 medication and become so severely malnourished that they need to be hospitalized,” said Rebecca Boswell, PhD, director of the Penn Medicine Princeton Center for Eating Disorders, in Philadelphia. “It’s not uncommon.”