Four out of five patients who said they had sex with their psychotherapists after therapy ended suffered psychological harm as a result, according to a study to be published this fall in the journal Psychotherapy.

The finding may bolster an emerging movement to ban sexual relationships between therapists and clients "in perpetuity." While sexual relationships are currently forbidden in all states during therapy, only Florida has an in perpetuity ban. A few states limit such relationships for varying periods after therapy.

National surveys have shown that up to 12 percent of male therapists and 4 percent of female therapists have been sexually intimate with current or former patients.

Study authors Ken Pope, a Los Angeles psychologist and author, and Valerie Vetter, senior special investigator for the Medical Board of California, found post-therapy intimacies can trigger an array of psychological problems for the former patient, ranging from nightmares, alcohol and drug abuse to difficulties forming intimate relationships.