Fifty-nine percent of U.S. mental health drug prescriptions are written by family doctors, not psychiatrists, raising concerns about the quality of some treatments, according to a study released on Wednesday.

Researchers from Thomson Reuters and the U.S. Substance Abuse and Mental Health Services Administration examined 472 million prescriptions written for psychotropic drugs from August 2006 and July 2007.

They found that general practitioners prescribed the bulk of prescriptions in two main categories -- 62 percent of antidepressants and 52 percent of stimulants.

The stimulants were mainly drugs for treating attention deficit hyperactivity disorder, or ADHD. About 25 percent of all stimulant prescriptions examined were written by pediatricians, they reported in the journal Psychiatric Services.

Family doctors also wrote 37 percent of prescriptions for drugs used to treat psychosis and prescribed 22 percent of anti-mania medications, the study showed.

The researchers said the findings are important because published independent research suggests that most people treated for depression are more likely to get adequate care in specialist psychiatric settings than from a family doctor.

The main concern is whether people are getting followed up adequately, said Tami Mark, director of analytic strategies for the healthcare and science business of Thomson Reuters, the parent company of Reuters.

You need to make sure people are coming in after you start them on these medications, to see if they're having adverse effects. A lot of people may not be getting followed up when they're seeing their GP, Mark said.

The researchers were startled to find that 37 percent of antipsychotic drugs were prescribed by general practitioners.

The fact that antipsychotics may be more complex to prescribe, have some potentially serious side-effects, further emphasizes the need to understand the adequacy of care being provided by a GP, Mark said.

At least 27 million Americans take antidepressants, more than double the number who did in the mid-1990s, according to a study published last month in the Archives of General Psychiatry.

Drugs called selective serotonin reuptake inhibitors, which affect the brain chemical serotonin, like GlaxoSmithKline's Paxil and Eli Lilly and Co.'s Prozac are the most commonly prescribed.

The U.S. Food and Drug Administration has issued its strongest warning -- the so-called black box warning -- on the use of all antidepressants because of a higher risk of suicide among patients under 25.

Millions of people take ADHD drugs including Novartis AG's Ritalin and Shire Plc's Adderall and Vyvanse.

Calls to poison control centers for U.S. teenagers who overdose on ADHD drugs have also soared, although the reasons are unclear.

The FDA this year approved the use of three widely used antipsychotic drugs for children -- Lilly's Zyprexa, AstraZeneca's Seroquel and Pfizer's Geodon.