The findings come as blood agencies increasingly rely on young people to maintain an adequate supply. Blood donation has declined in recent years, particularly among some older age groups, and the American Red Cross, which conducted the study, has supported efforts to allow more high school students to donate.

Lead author Dr. Anne Eder, an executive medical officer at Red Cross headquarters in Washington, D.C., emphasized that while teens are more susceptible to problems, their risks for having a bad reaction are still small.

"We want donors to know what to expect and we want them to have a good experience," Eder said.

Lightheadedness, sweating and small needle-related bruises were the most common problems. More serious complications were mostly caused by fainting ¡ª concussions, cuts and one broken jaw ¡ª and occurred much more often in the youngest donors.

Response to stress
It's unclear why teens are more vulnerable but Eder said they may respond differently to stress than adults. Low weight and small stature sometimes also play a role.

Almost 15 percent of annual blood donations nationwide come from donors aged 16 to 19. Most states allow 17-year-olds to donate blood; almost half allow 16-year-olds to donate with parents' consent, the study authors said.

The researchers analyzed 1.7 million donations collected in 2006 in nine states and Puerto Rico. Complications occurred with almost 11 percent of donations by 16- and 17-year-olds and about 8 percent of donations by 18- and 19-year-olds, compared with almost 3 percent in donors aged 20 and older.