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A new weight loss drug has been approved that promises to be the most effective medication yet. How is Zepbound, the weekly injectable drug that’s expected to be available shortly after Thanksgiving, different from other weight loss medications?  
Zepbound, from drugmaker Eli Lilly, belongs to a new class of medications, called GLP-1 agonists, that have skyrocketed in popularity in the U.S. in recent years. Novo Nordisk’s Ozempic and Wegovy and Lilly’s Mounjaro are the same type of drugs.
Ozempic and Mounjaro are prescribed to help people with diabetes control their blood sugar levels.
Wegovy and Zepbound are for people who don’t already have diabetes but whose excess weight puts them at risk of health complications. 
In clinical trials, Zepbound helped people lose, on average, 52 pounds, nearly rivaling the weight loss results achieved with bariatric surgery, according to Dr. Christopher McGowan, a gastroenterologist who runs a weight loss clinic in Cary, North Carolina. 
Here’s what you need to know about Lilly’s new drug. 
The active ingredient in Zepbound is tirzepatide, which mimics a hormone that helps reduce food cravings. The diabetes medication Mounjaro also contains tirzepatide.
Ozempic and Wegovy contain semaglutide, an ingredient that can suppress appetite by mimicking a hormone that signals to the brain when a person is full.
What makes tirzepatide even more helpful is it also imitates a second hormone, called GIP, which — along with reducing appetite — may also improve how the body breaks down sugar and fat, helping to amplify the weight loss effects.
In a phase 3 clinical trial, a 15-milligram dose of tirzepatide, the highest dose, led to an average weight loss of 22.5% body weight, or about 52 pounds. The participants in the study had obesity or were overweight with at least one weight-related condition.
At lower doses, patients also lost weight: At a 10 mg dosage, the average weight loss was about 21.4%, or about 48 pounds; at 5 mg, patients achieved an average weight loss of around 16%, or about 35 pounds.
In similar trials, semaglutide was shown to reduce body weight by around 15%, or about 34 pounds, after 68 weeks.
To be sure, these are not direct comparisons because the drugs were not compared in a head-to-head clinical trial.
The Food and Drug Administration approved Zepbound for use by adults with a body mass index, or BMI, of at least 30 or a BMI of 27 or more if a patient also has another weight-related condition, such as high blood pressure, heart disease or obstructive sleep apnea. 
The agency said the drug should be used along with a reduced calorie diet and exercise.
People who are prescribed the medication should not take another GLP-1 agonists, like Ozempic or Wegovy.
Zepbound is only approved for adults 18 and older. Wegovy is approved for people 12 and older. On Zepbound’s label, the FDA notes that the safety and effectiveness of Zepbound have not yet been established in teens or adolescents.
McGowan stressed that both medications are highly effective. When deciding between Zepbound and Wegovy, he said patients should consider two factors: insurance coverage and their individual response to each treatment.
Lilly has priced Zepbound at about $1,060 for a month’s supply, and while it’s less than Wegovy, which is priced at around $1,300, both drugs may be too expensive for many people who are eligible. Insurance companies often refuse to cover weight loss medications. Medicare, by law, does not cover them.
“Cost will be a barrier,” said Dr. Nishant Shah, a preventative cardiologist at Duke University Medical Center in Durham, North Carolina.
Eli Lilly said patients can sign up on its website for a copay, or a discount card program that could help lower the cost of the drug.
The company also said that people who are able to get Zepbound through commercial insurance may be eligible to pay as low as $25 for a one-month or three-month supply. People who are commercially insured but do not have coverage for Zepbound may be eligible to pay as low as $550 for a one-month prescription of Zepbound, according to the company.
During a briefing Wednesday, the company said it is working with insurers to help more people be able to afford the medication. 
Zepbound, like other GLP-1 agonists, can cause uncomfortable side effects, including gastrointestinal problems.
According to the FDA’s drug label, side effects include:
Other notable side effects include:
Generally, people experience those side effects as they ramp up the dosage at the start of treatment, but the side effects usually go away after prolonged use, according to Dr. Daniela Hurtado Andrade, an endocrinologist at the Mayo Clinic. 
People who are pregnant should not take the medication. The FDA said patients should also be monitored for depression or suicidal thoughts, and should stop taking Zepbound if symptoms develop.
While the Zepbound label does not explicitly mention the risk of stomach paralysis, also called gastroparesis, some patients have reported this complication from other weight loss medications.
Doctors in the U.S. should be able to prescribe Zepbound sometime after Thanksgiving, although Lilly executives didn’t say how many doses would be available this year.
Given the anticipated high demand, it’s unknown what kinds of shortages people who want the drug will face. The supply is expected to increase through 2024, Lilly said during the Wednesday briefing.  
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Berkeley Lovelace Jr. is a health and medical reporter for NBC News. He covers the Food and Drug Administration, with a special focus on Covid vaccines, prescription drug pricing and health care. He previously covered the biotech and pharmaceutical industry with CNBC.