If your idea of a relaxing weekend involves bingeing on your favorite TV shows, watch out!

You probably know that couch-potato TV watching increases risk for obesity and chronic conditions such as heart disease and diabetes, which are tied to a lack of physical activity. But new research has revealed another life-threatening—and surprising—risk from prolonged TV watching.

Recent finding: In research involving more than 15,000 middle-aged adults (ages 45 to 64), the risk of developing for the first time a blood clot in veins of the legs, arms, pelvis and/or lungs, a condition known as venous thromboembolism (VTE), was 1.7 times higher in those who reported that they watched TV “very often” than in those who reported “never or seldom.” VTE, which can be life-threatening if the clots break free and block arteries of the lungs, is more common in people age 60 and older but can occur at any age.

Surprisingly, the elevated risk was seen even in study participants who met recommended guidelines for physical activity (150 minutes of moderate aerobic exercise or 75 minutes of vigorous aerobic exercise per week) but reported watching TV “very often.”

Why would people who get their hearts pumping regularly also be at risk? “Watching TV itself isn’t likely bad, but we tend to snack and sit still for prolonged periods while watching,” explains Mary Cushman, MD, MSc, coauthor of the study and professor of medicine at Lamer College of Medicine, University of Vermont, in Burlington.

Here’s what happens: Most blood clots that develop due to inactivity occur in the legs. The blood has to travel back to the heart by working against gravity to move up the legs. So, if you are seated for a long time and your muscles aren’t working to keep the blood moving, clots can form.

This phenomenon is most common after an injury that lays you up in bed, but it can also happen during long periods of sitting in one position—for example, when you’re binge-watching all seven seasons of Game of Thrones.

Interestingly, even though obesity was more common in the study participants who watched more TV, only about 25% of the increased risk for VTE could be explained by being overweight, suggesting that something else is at work. Sitting still for lengthy periods of time is the likely culprit.

Are you at risk? Discuss your risk for VTE with your doctor, especially if you’ve had a previous blood clot…undergone surgery or suffered trauma (such as a serious fracture) within the past three months…are pregnant or have recently given birth…or have cancer—all of which increase VTE risk. In addition to curbing your couch time, your doctor may want you to take a blood-thinning medication or wear compression stockings to help protect you from developing clots.

Bottom line: The risks of watching too much TV are very real. If you can’t tear yourself away from those favorite shows, put a treadmill or stationary bike in front of your TV and keep moving while watching. Or stand up and move around for about five minutes for every half hour of TV watching.

If you have the ability to record your shows, you also can reduce your TV time by playing them back later and skipping the commercials. Even better: Choose a few days each week to turn off the TV and take a walk with some friends!