CHICAGO – Children on widely used psychiatric drugs can quickly gain an alarming amount of weight; many pack on nearly 20 pounds and become obese within just 11 weeks, a study found.

“Sometimes this stuff just happens like an explosion. You can actually see them grow between appointments,” said Dr. Christopher Varley, a psychiatrist with Seattle Children’s Hospital who called the study “sobering.”

Weight gain is a known possible side effect of the anti-psychotic drugs that are prescribed for bipolar disorder and schizophrenia, but also increasingly for autism, attention deficit disorders and other behavior problems. The new study in mostly older children and teens suggests they might be more vulnerable to weight gain than adults.

The study also linked some of these drugs with worrisome increases in blood fats including cholesterol, also seen in adults. Researchers tie these changes to weight gain and worry that both might make children more prone to heart problems in adulthood.

The research is the largest in children who had just started taking these medicines, and provides strong evidence suggesting the drugs, not something else, caused the side effects, said lead author Dr. Christoph Correll of North Shore-Long Island Jewish Health System in Glen Oaks, N.Y.

But because these drugs can reduce severe psychiatric symptoms in troubled children, “We’re a little bit between a rock and a hard place,” he said.

The study authors said their results show that children on the drugs should be closely monitored for weight gain and other side effects, and that when possible, other medicines should be tried first.

The study appears in today’s Journal of the American Medical Association. It involved 205 New York City-area children from 4 to 19 years old who had recently been prescribed one of the drugs; the average age was 14.

Depending on which of four study drugs children used, they gained between about 10 and 20 pounds on average in almost 11 weeks; from 10 percent to 36 percent became obese.