A review of thousands of patient records has turned up a previously unknown risk associated with a popular weight loss operation — suicide.

In bariatric surgery, the stomach is made smaller so as to speed weight loss. The risk of dying from bariatric surgery is about 1 ipercent, most studies show, and complications strike up to 40 percent of patients. In addition to being overweight, these patients often have health problems like diabetes and heart disease, so it’s no surprise they also have higher death rates from natural causes.

But a review of nearly 17,000 weight-loss surgeries performed in Pennsylvania from 1995 to 2004 has yielded a surprising finding. Of the 440 deaths in the group, 16 were due to suicide or drug overdose, according to the University of Pittsburgh researchers who reviewed the data. Based on the suicide rate in the general population, no more than three suicides should have occurred in the group, the study authors say. More troubling is the fact that another 14 of the drug overdoses that were reported likely include some suicides, suggesting that the real suicide rate was even higher. “There is a substantial excess of suicide deaths, even excluding those listed only as drug overdose,'’ the researchers noted.

In August, The New England Journal of Medicine reported a review of nearly 10,000 bariatric surgery patients by Utah researchers, who compared them to a control group of obese people who had applied for a state driver’s license. Although the surgery patients had a 50 percent lower risk of dying from disease compared to obese people who hadn’t undergone surgery, their risk of dying in an accident or suicide was 11.1 per 10,000 people — that’s 58 percent higher than the 6.4 per 10,000 rate in the obese group. The study suggested the suicide risk was twice as high for surgery patients than for those who had not had surgery, but the finding wasn’t statistically meaningful.

Nobody knows why bariatric surgery patients appear to be at higher risk for killing themselves. Some research shows a link between obesity and depression, so the typical surgery patient may already be at higher risk for depression and suicide before the operation. It’s possible that depressive symptoms may worsen in patients who have unrealistic expectations about the results of surgery or who struggle not to regain weight after the procedure.

The study authors say the 7 percent death rate from suicide and drug overdose in the Pennsylvania case review signals the need for better mental health follow-up for patients who have undergone weight-loss surgery. While most weight-loss surgery programs require some psychological evaluation before the procedure, many programs and doctors are lax about follow-up after the surgery, and patients themselves often opt to skip follow-up counseling.