A Harvard Medical School psychiatrist warned a national science conference Thursday that there is a grave danger in the growing medical tendency to prescribe Valium and Librium tranquilizers "to save the doctor time and avoid the reality of a patient's emotional problems."

In an address to the American Association for the Advancement of Science in Boston, Dr. Gerald L. Klerman, professor of psychiatry, attributed the growing dispension of tranquilizers to "the expanding definition of illness in our society."

"The Declaration of Independence guarantees us life, liberty and the pursuit of happiness. Today people expect these rights to be covered by Blue Cross," Klerman said yesterday.

Tranquilizer use is particularly prevalent among the middle-aged, and twice as common among women as men, Klerman said. His data includes figures on tranquilizer use in sex therapy, cosmetic surgery and contraception, as well as in psychiatry, he said.

"Younger people and younger men in particular tend to use stiumulant drugs and alcohol instead of tranquilizers," Klerman said.

Dr. Warren O. Wacker, director of the University Health Services said yesterday that Harvard doctors have no written policy governing Valium and Librium prescriptions, but that "they don't throw it around."

Klerman said he has no data on student use of tranquilizers.