Someone in America needs blood every two seconds, according to the American Red Cross. But while the need for blood continues to grow, the number of people offering to donate is declining. And a new study shows that an important source of current and future donations… sixteen and seventeen year olds… is more likely to experience complications while giving blood, such as fainting or bruising.

The study, published in the Journal of the American Medical Association, found complications occurred in nearly eleven percent of sixteen and seventeen year olds. That’s compared to nearly three percent for adults aged twenty and older.

The study also found that first-time donors were more than three times as likely to experience complications compared to repeat donors. Also, female donors are nearly twice as susceptible to difficulties when giving blood.

Why is this important? Because these bad experiences can influence the willingness of young donors to give blood in the future. Only fifty-two percent of sixteen year olds who experienced a problem returned for a repeat donation within a year, compared with seventy-three percent of those whose donation went smoothly.

This is worrisome because the need for blood is rising, researchers say. Between 2001 and 2004, there was a point-two percent decrease in blood donations in the U-S, while the number of transfusions rose by two percent.

So what can be done to ensure a smoother donation? Doctors recommend drinking lots of water, getting a good night’s sleep and eating a healthy meal beforehand.