Just one episode of dangerously low blood sugar might increase the risk of death, heart disease and stroke in people with type 2 diabetes, a new study suggests.

The low blood sugar (hypoglycemia) episode had to be so serious that it required a visit to the emergency room, according to researchers from Johns Hopkins Bloomberg School of Public Health in Baltimore.

"If you have a patient with a history of severe hypoglycemia, this could portend poorly for his or her future," said study co-author Alexandra Lee, a Ph.D. candidate in epidemiology.

"Our thinking has been that you resolve a hypoglycemic episode and it's over. But what this tells us is that one episode may have long-lasting consequences," Lee said in a school news release.

However, the researchers also noted that it's not clear from this study whether people who had a severe low blood sugar episode were already sicker than those who didn't have such an episode, or if it was the low blood sugar episode that actually contributed to the higher risk of death, heart disease and stroke.

The research included nearly 1,200 adults with type 2 diabetes who were followed for an average of 15 years. They were between 45 and 64 years old.

Almost 200 of those in the study had a low blood sugar episode that required an ER visit. One-third of those people died within three years of that episode.

During the study period, the risk of heart disease and stroke or death was twice as high among patients who'd had severe hypoglycemia than among those who had not, the researchers found.

The results suggest that doctors should pay close attention to diabetes patients who have received emergency department treatment for hypoglycemia after serious events such as losing consciousness or having a seizure, the researchers said.

"Hypoglycemia is clearly an under-recognized risk factor for death and cardiovascular disease in people with diabetes," said study senior author Elizabeth Selvin, a professor of epidemiology at the school.

"We are treating many people in this country for high blood sugar. They need to be very careful that their treatment doesn't go too far and cause hypoglycemia, a potentially more serious condition than we have truly understood," Selvin said.

About 29 million Americans have diabetes, the researchers said. An estimated 85 percent take medications to lower their blood sugar.

The study findings are scheduled to be presented Friday at an American Heart Association meeting in Portland, Ore. Findings presented at meetings are usually viewed as preliminary until they've been published in a peer-reviewed journal.