Perfect Little Angels (1998)

Having just moved to a new town, widow Elaine Freedman and her teenage daughter Justine are greeted by the effusive, terribly nice Dr. Calvin Lawrence. Sensing that Justine is bit testy and out of sorts, Dr. Lawrence recommends that she begin taking a special vitamin which he manufactures in his own home. Before long, Justine is the model of perfection -- just like all the other teenagers in town.