A man rapidly lost weight on a popular weight-loss drug. After 6 months, he was hospitalized with heart problems and confusion.

• A man was hospitalized with serious health problems after losing 80 lbs in 6 months.
• He was taking tirzepatide, a popular weight-loss drug, as well as a thyroid medication.

Six months after he started taking a weight-loss drug, a 62-year-old man was hospitalized with heart palpitations, tremors, confusion, fever, and sweating.

The man, from Colorado, had been prescribed the drug tirzepatide (the active ingredient in Mounjaro and Zepbound) to lose weight. It was working — he had dropped more than 80 pounds, and gradually increased the dose over time.

But he was also taking other medications, for an autoimmune condition and for type 1 diabetes, and missed a follow up appointment with his doctor. Over time, the patient decreased his insulin dose as he lost weight, but didn't change his thyroid medication.

When he checked into the ER, doctors determined that losing weight too quickly, without adjusting his other medication, triggered a dangerous thyroid condition called thyrotoxicosis.

Patients on multiple medications, especially weight-based drugs, need careful monitoring while taking tirzepatide, his doctors from the Barbara Davis Center for Diabetes at the University of Colorado wrote in his case report, which was published August 5 in JAMA Internal Medicine. (They withheld the patient's name for privacy.)

While ths is a rare instance of a serious health issue linked to the popular new class of medications called GLP-1s his doctors say this case holds lessons for all patients. This highlights the potential risks of losing weight too quickly, and the need for medical supervision while using the drugs, they said.

GLP-1 medications for weight loss have exploded in popularity

Drugs like tirzepatide and a similar medication called semaglutide were initially approved and sold to treat diabetes, but were FDA-approved for weight loss after research found they led to groundbreaking results.

Tirzepatide, sold as Mounjaro and Zepbound, helped clinical trial patients lose more than 24% of their body weight (64 pounds on average) over 84 weeks.

Ozempic and Wegovy, brand names for semaglutide showed weight loss of about 15% over a similar time period.

Doctors have called GLP-1 medications a game-changer for treating potential health issues that can arise from a higher weight and high levels of body fat. A growing number of people are taking the medications. Researchers estimate that even more people who could benefit from the drugs aren't able to access them due to cost (as much as $1,000 per month) and shortages.

Fast weight loss causes side effects like 'Ozempic face' and loss of muscle

GLP-1 medications work by reducing appetite and slowing digestion, and can sometimes cause nausea, diarrhea, and constipation

But they've also been linked to other potential issues including so-called "Ozempic face" (sagging skin), loss of muscle, and hair loss.

However, those symptoms aren't uniquely related to the drugs themselves, but to the physical consequences of rapidly cutting calories and dropping pounds.

Losing weight too quickly, whether by crash dieting or as a consequence of meds, can increase the odds of losing muscle mass, doctors previously told BI. Depriving your body of energy can lead to side effects like fatigue and temporary hair loss.

As a result, doctors advise patients to not cut calories too low even if they've lost appetite on the medications, and aim to eat enough protein to help protect muscle tissue.

More serious side effects like blindness and stomach paralysis have also been reported, but are rare.