Children taking popular psychiatric drugs are experiencing stunning weight gain – as many as 20 pounds – and are often becoming obese within the first three months of use, new research indicates.

In the largest study ever of children starting so-called second-generation antipsychotics, researchers report Tuesday that they saw an increase of nine to 19 pounds for the four most commonly prescribed psychiatric medications after just 11 weeks of use.

That’s “massive, rapid weight gain,” according to psychiatrist Christoph U. Correll who led the study published in the Journal of the American Medical Association. It’s particularly alarming because researchers believe childhood weight gain may cause heart problems early in adulthood.

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Researchers looked at 272 kids ages 4 to 19 who had never taken antipsychotics before. Nearly half had mood spectrum disorders, while 30 percent had schizophrenia and roughly 20 percent had disruptive or aggressive behaviors. Fifteen children who were advised to use antipsychotics but chose not to made up a control group.

In just 11 weeks on the drugs, as many as 36 percent of children became overweight or obese.

On average children taking Zyprexa grew 18.7 pounds while those on Seroquel added 13.4 pounds. Risperdal patients gained an average of 11.7 pounds and children taking Abilify added 9.7 pounds. Meanwhile, kids in the control group gained just 0.4 pounds.

Why? The medications increase the appetite and likely also delay the feeling of fullness, Correll says. Over weeks or months of taking the medication, researchers believe that the body creates a new set point for food so that the weight gain eventually plateaus.

“If you say, ‘oh come on, I always had a normal weight so why should I change anything,’ you should change something because these medications alter something that you need to combat actively,” Correll says.

With that, researchers say that antipsychotics should only be used in patients who absolutely need them and only for as long as is necessary.

If antipsychotics are the best way to go, however, taking a proactive approach is your best bet to stave off some of the weight gain and ensure that your body’s new set point is as low as possible. How? Set some rules and stick to them.

Eat smart

Lots of small meals are better than a few big ones, Correll says, and always wait before taking seconds. That way you give your body time to feel full. But make sure not to skip an important meal like breakfast. It’s a must to keep a good metabolism going.

Dessert on the other hand is a big no-no, unless you limit it to fruit. Sugary drinks are also out, says Correll, even diet sodas and juices.

“Our brain is not made to count liquid calories as much as things we chew because for millions of years there was only water. Unsweetened tea and water is allowed and a moderate amount of milk,” Correll says.

The key is not to get fooled. Diet soda and fat free foods seem like a good idea, but they actually have a tendency to make you feel hungrier.

And as always, avoid fast food, but keep the fruits and vegetables coming.

Get Moving

Exercise is essential. A bare minimum of 20, if not 30, minutes of exercise everyday is a must. Even moderate walking will help.

Self-monitor

Pay attention to your weight and talk to your doctor about any weight gain that you see. In many cases there are other options if antipsychotics are forcing you to constantly loosen your belt buckle. For example, it’s important to treat underlying conditions through behavioral therapy and social skills groups, Correll says.

If medication is the only option, make sure you’re on the lowest dose possible and consider other classes of drugs, like mood stabilizers, that can have similar benefits without the weight gain side effect.

“We have to balance the severe effects and dangers of the serious illness against the side effects of the medication. Kids that cannot go to school that cannot interact with family and peers, that can’t reach their developmental milestones, that can’t learn what’s needed to function as an adult, then this is a psychiatric emergency,” Correll says. “The problem is that obviously these treatments have side effects so we have to select these medications carefully.”