CHICAGO – Complications from donating blood, including dizziness, fainting and falls, are rare but happen more often in teens than in older donors, a study found.

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 – concussions, cuts and one broken jaw – and occurred much more often in the youngest donors.

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.

Young people who start donating blood early tend to continue throughout their lives, said Dr. Peter Tomasulo, chief medical officer for Blood Systems Inc. The Scottsdale, Ariz. -based company oversees blood centers in 18 states.

“We are constantly trying to make sure that no patient ever has to wait for a blood transfusion. It’s a very big struggle for us and these kids play a very big part,” Tomasulo said. He was not involved in the study, which appears in Wednesday’s Journal of the American Medical Association.

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.

High school sophomore Joe Gibson said the small risks shouldn’t deter teens. Gibson helped draft a new law in Minnesota allowing 16-year-olds to donate after seeing how blood transfusions benefited his grandfather, who had cancer.

The law takes effect in July and Gibson, 16, plans to participate in a blood drive soon after in his hometown, Blooming Prairie.

“It’s just a great thing to do, something that helps so many people,” he said.

The risks need to be weighed against the dangers of not having enough blood for patients who need transfusions, said Dr. Ram Kakaiya, medical director for LifeSource, which conducts blood drives at about 300 Chicago-area high schools each year.

Donors are encouraged to drink plenty of fluids and are fed snacks and urged to stick around for several minutes afterward to help avoid complications, Kakaiya said.