Harvard Medical School, in an effort to make it more affordable to become a doctor and increase the number of graduates choosing lower-paying specialties, announced yesterday that it will dramatically cut costs for nearly one-third of its students.

The plan, which follows tuition-reduction efforts for Harvard law students and undergraduates and at other top universities around the country, will reduce the cost of a four-year medical education by as much as $50,000 for families with an income of $120,000 or less.