People just starting an antidepressant (search) should be aware that it may increase your risk of bleeding, according to new research.

Researchers say the risk of bleeding associated with antidepressants is due to the brain chemical serotonin (search), a chemical messenger that plays a role in depression and anxiety.

Most antidepressants affect serotonin. Some antidepressants, such as selective serotonin reuptake inhibitor (SSRI) antidepressants, including Paxil, Prozac, and Zoloft, affect this chemical more strongly than other antidepressants.

The finding is published in today’s issue of The Archives of Internal Medicine.

In addition to affecting mood, serotonin also plays a role in blood clotting. Previous studies have shown an association between the use of SSRIs and abnormal bleeding, particularly excessive uterine bleeding and stomach and intestine bleeding. But the evidence has been considered inconclusive.

For the new study, Welmoed E. E. Meijer, PhD, and colleagues in The Netherlands examined 64,000 medical records of patients that had taken antidepressants. Patients were considered “new” users if they had a prescription for at least a year but no history of prior antidepressant use.

There were 196 cases of abnormal bleeding. Nearly half were hospitalized for abnormal uterine bleeding. Stomach and intestinal bleeding accounted for 16 percent, brain hemorrhages accounted for 10 percent, and abnormal bleeding in joints, nose bleeds, and bleeding within the bladder, accounted for almost one-fifth of bleeding requiring hospitalizations.

Risk of hospitalization increased with the use of drugs that more strongly affected serotonin. Antidepressants generally work by increasing exposure of serotonin and other chemicals to the brain cells.

Those patients taking antidepressants with the strongest affect on serotonin had a 2.6-fold higher risk of abnormal bleeding compared with those taking antidepressants with the lowest effect. Antidepressants with an intermediate effect nearly doubled the risk.

SSRI antidepressants with the greatest serotonin effect include Prozac, Zoloft, and Paxil. Anafranil, another type of antidepressant used to treat obsessive compulsive disorder, also has a strong effect. Antidepressants with an intermediate serotonin effect include Effexor and the SSRI antidepressant Celexa. Remeron, Serzone, Doxepin, and Wellbutrin have a low affect on serotonin.

This is a preliminary study based on medical records only. If you are taking an antidepressant, talk with your health care provider to discuss the benefits and any risks of your medication. Do not stop taking your antidepressant without first talking to your health care provider.

By Kelli Miller Stacy, reviewed by Brunilda Nazario, MD

SOURCES: Meijer, W. Archives of Internal Medicine, 2004; vo 164: pp 2367-2370. News release, JAMA.