Obese people have a choice of two powerful weight-loss drugs. One is semaglutide, marketed by Novo Nordisk as Wegovy for obesity and as Ozempic for diabetes. The second, tirzepatide, is sold by Eli Lilly as Zepbound for obesity and as Mounjaro for diabetes. Many people who are neither obese nor diabetic take these drugs to lose weight.
A recent study suggests that people taking Mounjaro lose more weight than those taking Ozempic: Which one should I take? And if I’m already taking one of them, should I switch?
According to obesity medicine experts, the answers are not so simple. Here are some factors that can help distinguish hype from realistic hope.
Is one weight loss drug really better than another?
For now, it’s hard to say. All the information available comes from “very poor studies,” says Dr. Diana Thiara, medical director of the weight-loss clinic at the University of California, San Francisco.
That includes the recent study comparing Mounjaro and Ozempic. Using electronic medical records, researchers reported that those taking Mounjaro lost an average of 15.3% of their weight after one year. Those taking Ozempic lost an average of 8.3%.
As shocking as it may sound, Dr. Susan Z. Yanovski, co-director of the Office of Obesity Research at the National Institute of Diabetes and Digestive and Kidney Diseases, said, “I wouldn’t make any decisions about my health care based solely on a study like this.”
There is an inherent difficulty in using electronic health records, he noted, because it is not known why patients were taking the drugs — the study was underway before Zepbound was approved for treating obesity. Researchers looked at prescriptions for Ozempic and Mounjaro, approved to treat diabetes. However, many of the study participants did not have diabetes.
The researchers also didn’t know the dosages — they just knew that a prescription had been filled. That’s a problem because the drugs can be prescribed at different doses, which “can lead to different weight loss outcomes,” said Tricia Rodriguez, a senior applied scientist at Truveta Research and principal investigator of the comparative study.
And, Dr. Yanovski noted, fewer than half of the patients were still taking the drugs at the end of the study.
It’s also true that clinical trials of the two drugs showed differences in weight loss. Novo Nordisk reported that participants taking Wegovy lost 14.9 percent of their body weight after 68 weeks. For Zepbound, Eli Lilly said participants lost 20.9 percent after 72 weeks at a 15-milligram dose. But these weren’t studies comparing the drugs against each other, so it’s difficult to compare the results.
Future research will provide better answers: Eli Lilly is conducting a clinical trial comparing Zepbound to Wegovy in people with obesity who do not have diabetes. It is expected to be completed later this year.
Why did my doctor choose one medication over another?
It may depend on your health insurance. According to Dr. Thiara, some insurers will pay for one but not the other. Others, she added, will insist that patients start on Wegovy and will only pay for Zepbound if they don’t lose weight or if the side effects are intolerable.
Dr. Thiara has prescribed both drugs to patients, but says her choice is often determined by a patient’s insurance.
“Insurance companies dictate a lot of what we can do,” says Dr. Thiara.
Or your doctor may be being cautious. Dr. Yanovski noted that Wegovy has been on the market longer.
Another consideration might be other medical conditions for which the drugs might be helpful.
Semaglutide reduces the risk of cardiovascular disease. This has not yet been proven for tirzepatide. The drug also reduces complications in people with kidney disease. Novo Nordisk found in another clinical trial that Wegovy improved physical functioning – such as the ability to exercise – in people with obesity and heart failure.
On the other hand, Eli Lilly found that Zepbound could help with sleep apnea. Patients taking the drug also had significantly lower blood pressure.
Several clinical trials are testing the effects of semaglutide and tirzepatide on even more diseases: cancer, arthritis and even Alzheimer’s.
What if I want to change medications to lose more weight?
If you’re doing well on Wegovy and your health has improved, you may want to stick with it, said Dr. Rudolph Leibel, a diabetes and obesity researcher at Columbia University Irving Medical Center. It’s not clear that switching to Zepbound is better.
“On the other hand,” Dr. Leibel said, “it is clear that there are people who do not respond well to semaglutide and may benefit from switching to tirzepatide.”
But it is virtually impossible to predict who will respond best to one of these drugs and who will experience fewer side effects.
Health is also an issue.
If a patient is healthier after starting Wegovy, with changes such as lower blood pressure and reduced amounts of fat and inflammation in the liver, for example, that’s an argument for continuing to take the drug, Dr. Yanovski said.
Of course, Dr. Thiara said, many patients want to lose as much weight as they can — many have struggled with obesity and its stigma for years and just want it to end. If switching to Zepbound helps them lose more weight, they want to do it.
Dr. Thiara says she emphasizes goals with her patients: the goal is to be healthy, “not a size 2.”
“If your goal is to look a certain way, there’s not much I can do,” he adds.
Are there any disadvantages to the change?
When patients start taking an anti-obesity drug, their doctors often prescribe a low dose and then increase it. The goal is to minimize side effects, such as nausea and vomiting.
According to obesity medicine experts, there is no data to suggest that switching from one drug to another at a higher dose would cause problems. But many doctors are cautious and want to give patients at least a month on the lower dose of the new drug to see how it is tolerated.
However, by switching to a lower dose, some weight may be regained.
Another question to consider is whether it will be easier for you to get one drug than the other. Because demand is so high and production is stretched, both drugs can be hard to come by and their availability “comes and goes,” says Dr. Thiara.
Will there be more weight loss drugs in the future?
The drugs being developed promise to be even more potent than the two currently on the market. Economists expect that with more drugs, prices will come down and supply restrictions will ease.
Doctors expect the decision of which drug to prescribe will become even more complex.
But that’s a good problem to have, Dr. Leibel said, adding that it reminded him of the evolution of high blood pressure drugs. At first there were only a few, and they were less effective and had more side effects than the dozens of blood pressure drugs available today. Now most patients can find a drug or combination of drugs to control their blood pressure with no or minimal side effects.
Anti-obesity drugs, he predicted, are going the same way.
“This is a new turning point in the history of obesity treatment,” said Dr. Leibel.
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