Obesity is a major and growing problem around the world, but especially in the U.S., where more than 40% of adults and about 20% of children now meet the criteria for what doctors say has become an intractable chronic disease.

Rates of the disease have soared in recent decades, spurred by the complex interaction of genes that make people more likely to store food as fat, a food system that provides easy and cheap access to processed treats explicitly designed to be overconsumed, and social settings that limit access to healthy options and exercise for many people.

Obesity is linked to scores of health problems that can lead to disability or even death, including high blood pressure, diabetes, heart disease, stroke, cancer and joint problems.

Researchers have long looked for medications that can help people lose weight, mostly with disappointing and, in some cases, dangerous results. In recent years, however, drugs designed to help people with type 2 diabetes control their blood sugar levels have had the added effect of paring pounds.

Ozempic, a Novo Nordisk drug approved to treat diabetes in 2017, skyrocketed in use after celebrities and ordinary people on TikTok reported that their doctors prescribed it “off label” for weight loss. Wegovy, a higher dose version of the same medication, called semaglutide, was approved for weight loss for adults in 2021 and for children aged 12 and older late last year.

Now, a new drug made by Eli Lilly and Co., called tirzepatide, is poised to become the most potent obesity drug on the market, promising users losses of more than 30 to 50 pounds over time. Already approved under the brand name Mounjaro to treat type 2 diabetes, tirzepatide is being considered for fast-track approval as a weight-loss drug based on the results of key trials, with the latest announced on Thursday.

The new study found that patients with diabetes — who find it notoriously difficult to lose weight — could shed about 16% of their body weight, or more than 34 pounds using tirzepatide. An earlier study found that people without diabetes lost up to 22% of their body weight, or more than 50 pounds on the highest dose of the drug.

Tirzepatide and other medications spur weight loss by targeting the metabolic conditions that lead to extra pounds. Here’s what to know about these new prescription drugs that promise weight loss. WHAT ARE THESE NEW WEIGHT LOSS DRUGS?

The drugs that have drawn the most attention have been a class of medications that activate a hormone known as GLP-1. They include Ozempic and Wegovy, which are two versions of the same medication, semaglutide.

Tirzepatide targets GLP-1, but also affects a second hormone, called GIP, which developers say contributes to its increased effectiveness. Mounjaro was approved to treat diabetes in May 2022.

The drugs are delivered through once-weekly injections. Users are advised to follow a healthful, reduced-calorie diet and to exercise regularly while using the drugs.

HOW DO OZEMPIC, WEGOVY AND MOUNJARO WORK?

The drugs work by mimicking the actions of hormones, found primarily in the gut, that kick in after people eat. The hormones help regulate blood sugar by triggering the pancreas to release insulin, another hormone, and slowing the release of sugar from the liver. People who are overweight or have obesity can become insulin-resistant, which means the body doesn't respond to insulin properly.

The obesity drugs lower blood sugar and slow down digestion, so people feel full longer. They also affect signals in the brain linked to feelings of fullness and satisfaction, tamping down appetite, food-related thoughts and cravings.

Because people feel full longer, they eat less and lose weight.

HOW EFFECTIVE ARE THE DRUGS?

In a trial, adults who took Wegovy saw a weight loss of nearly 35 pounds, or about 15% of their body weight. Adolescents lost about 16% of their body weight.

The latest study of tirzepatide studied the drug in more than 900 patients with diabetes who were overweight or had obesity over nearly 17 months. It showed weight loss of up to 16% of body weight, more than 34 pounds, when using the highest dose of the drug. Patients who received placebo, or dummy injections, lost about 3% of their body weight, or 7 pounds.

An earlier trial of tirzepatide showed weight loss of between about 15% and about 22% of body weight, or about 35 pounds to about 52 pounds, depending on the dose.

The drugs appear effective for chronic weight management over many months. In addition to weight loss, they also reduce health problems associated with obesity, such as high blood sugar and markers of heart and metabolic disease.

However, it appears that if people taking the drugs stop, they regain the weight they lost — and the health problems that came with it.

WHY NOT JUST DIET AND EXERCISE?

In a typical weight-loss program where participants rely only on diet and exercise, research shows only about a third of people will lose 5% or more of their body weight, said Dr. Louis Aronne, director of the Comprehensive Weight Control Center at Weill Cornell Medicine.

Many people find it difficult to lose weight because of the body’s biological reactions to eating less, he said. There are several hormones that respond to reduced calorie intake by ramping up hunger to maintain body mass.

WHAT ARE THE SIDE EFFECTS OF THE DRUGS?

The most common side effects are short-lived gastrointestinal issues such as nausea, vomiting, constipation, diarrhea and stomach pain. Other possible effects include serious issues such as inflammation of the pancreas, kidney, gallbladder and eye problems. People with a history of certain thyroid cancers or a rare, genetic endocrine disorder should avoid the drugs, because it is not clear if tirzepatide causes thyroid problems, including cancer.

HOW MUCH DO THESE DRUGS COST?

The new anti-obesity medications are expensive. Wegovy costs about $1,300 a month and Mounjaro starts at about $1,000 a month. People with private insurance may be able to receive the drugs with only a small co-payment. However, many insurers don’t pay for the medications or they have restrictions regarding coverage. Medicare doesn’t cover most weight-loss drugs. Medicaid and the military insurer Tricare may cover them in some cases with prior approval.