The first study to assess how weight-loss drugs affect health across humans has found an "eye-opening" effect of weight-loss drugs on the body, researchers say.

The analysis, involving about 2 million people, showed the drugs could improve heart health, reduce infections, reduce the risk of substance abuse and reduce cases of dementia.

US researchers also warned that the drugs were "not without risks" and appeared to increase joint pain and potentially fatal inflammation of the pancreas.

However, the results require very careful interpretation.

Weight-loss drugs have exploded in popularity, but a full understanding of what they touch in the body is still evolving.

"This is new territory," said lead researcher Dr. Ziyad al-Aly, a clinical epidemiologist at the University of Washington.

Initially, they were an effective treatment for type 2 diabetes. Then, weight loss was considered a significant side effect - and Ozempic and Wegovy became household names.

The study used data from U.S. veterans with type 2 diabetes, some of whom took Ozempic or Wegovy along with more standard medications, to measure their effects on 175 other conditions.

People taking the new weight-loss pill appear to have significant heart health benefits, with lower rates of heart disease, stroke, heart failure and high blood pressure.

They also reduce the risk of substance abuse, including alcohol, opioids, and marijuana, and reduce schizophrenia, suicidal thoughts, and seizures.

Although the study was short and people took the drugs for only 3.5 years because they were new, the study reported a 12 percent reduction in the incidence of Alzheimer's disease.

There were also reductions in liver cancer, muscle pain and chronic kidney disease, as well as significant reductions in bacterial infections and fevers.

On the other hand, people are more likely to have problems with their digestive systems. Feeling sick, tummy aches, stomach inflammation, diverticulitis (a bulge in the intestines that may be painful) and hemorrhoids are more common on Ozempic or Wegovy.

data, Published in Nature MedicineLow blood pressure, including fainting, headaches, restless sleep, kidney stones, kidney inflammation, and a range of bone or joint pains, including arthritis, become more frequent.

"It was absolutely eye-opening to see all these different hits in different organ systems," Dr Ali told BBC News.

The explanation for the seemingly widespread effects of these drugs is both obvious and mysterious.

Losing excess weight will in turn improve your health. For example, lower levels of sleep apnea (where breathing stops and starts while sleeping) are thought to be due to a loss of weight around the tongue and throat, which blocks the airway.

But these drugs also appear to directly change the behavior of cells and tissues in the body.

"Obesity is bad for the brain. Obesity is bad for mental health. Obesity is bad for the heart. Obesity may be the mother of all diseases," Dr. Ali said.

Ozempic and Wegovy have different doses of the same active ingredient, semaglutide, and mimic the hormone glucagon-like peptide-1.

GLP-1 is released from the intestines after eating, travels through the bloodstream and sticks to small receptors on the surface of brain cells.

This tells the brain that there is food in the stomach, which is why people feel less hungry after eating.

However, receptors that respond to GLP-1 are found throughout the body, including in the heart and parts of the immune system.

"Clearly, this class of drugs seems to suppress the reward mechanism (in the brain) and suppress the urge to seek out alcohol, seek out tobacco, seek out gambling," Dr. Ali said.

At the same time, lower levels of inflammation—the immune system’s wake-up call—can have widespread health effects.

Dr. Ali said a range of health benefits may strengthen the case for some people to use these drugs.

"When you add more benefits, it's an added benefit for people who are really at risk for these diseases," he said.

But for those whose weight doesn't affect their health, "perhaps the risks of their own purchases are actually far greater than the benefits."

However, the study had several flaws that limited its findings.

Most veterans are white men, so it doesn't include any female-specific effects, such as increased fertility and the unexpected anecdotal phenomenon of "ozone babies."

There may be a reason why some people take Ozempic or Weygovy instead of other drugs, which could provide alternative explanations for some of the findings.

Thorough clinical trials have demonstrated heart-health benefits — nausea is a known side effect — but other findings need to undergo similarly rigorous testing.

Alzheimer's disease can start more than a decade before symptoms appear, but this study shows that taking semaglutide for just a few years can produce a protective effect.

The test is Already in progress to determine whether this effect is real.

Professor Navid Sattar of the University of Glasgow said: "Trials like this will bring us closer to the truth."

"Fortunately ... several will be reported in the next one to four years."

While "interesting," he said the results of this latest study were not enough to influence how the drug is prescribed.

Sir Stephen O'Rahilly, a professor at the University of Cambridge, said the study required "careful" interpretation but provided "useful reassurance" about the safety of the drug for people with diabetes.

Further studies in other patients are also "awaited with interest."

"The most surprising finding" was an increase in joint pain, since losing weight should reduce stress on joints.

But the fact that some cells in the immune system have GLP-1 receptors means the effects of these drugs are "somewhat unpredictable," and while some inflammatory diseases may be relieved, "others may be exacerbated." , said Professor O'Rahilly.