Patients’ risk of suicidal behavior does not vary depending on the drug, according to a study of British medical records in Wednesday’s Journal of the American Medical Association. But patients’ risk is highest in the first month of taking a drug.

In the most comprehensive study of its type, researchers from the Boston University School of Medicine analyzed the computerized medical records of 159,810 patients ages 10 to 69 treated for depression between 1993 and 1999. They then compared the suicide risk of the antidepressant drugs Paxil and Prozac with that of older medications such as amitriptyline.

Researchers found patients were three to four times more likely to admit thoughts of suicide or suicidal attempts in the first month of treatment than they were after three months, regardless of age or sex.

Researchers said they do not know why the risk of suicidal behavior was highest during the first weeks of treatment, though patients are at an emotional nadir when depression is diagnosed and they begin treatment.