Few medicines, in the history of pharmaceuticals, have been greeted with as much exultation as a green-and-white pill containing 20 milligrams of fluoxetine hydrochloride — the chemical we know as Prozac. In her 1994 book “Prozac Nation,” Elizabeth Wurtzel wrote of a nearly transcendental experience on the drug. Before she began treatment with antidepressants, she was living in “a computer program of total negativity . . . an absence of affect, absence of feeling, absence of response, absence of interest.” She floated from one “suicidal reverie” to the next. Yet, just a few weeks after starting Prozac, her life was transformed. “One morning I woke up and really did want to live. . . . It was as if the miasma of depression had lifted off me, in the same way that the fog in San Francisco rises as the day wears on. Was it the Prozac? No doubt.”