We obtained information on drug use from the Ontario Drug Benefit Program, which provides comprehensive drug benefits to all residents aged 65 or older in Ontario, Canada. We identified all patients who had been taking lithium or valproic acid between 1993 and 2001 (prevalent users) and we further identified those patients who had not previously taken lithium or valproic acid (new users). We restricted our study to patients aged 66 or more to enable us to examine their previous drug use for a minimum of one year. Using unique encrypted health card numbers, we linked data on this cohort to two other large datasets—the Canadian Institute for Health Information dataset of all hospital separations and the Ontario Health Insurance Plan dataset of all claims for physician services. These datasets provided information on hospitalisations and visits to physicians that helped us distinguish between psychiatric and anticonvulsant uses of lithium and valproic acid and between bipolar disorder and other indications such as dementia.
Among patients who had no previous history of convulsive disorders, we identified 3902 patients who had started taking lithium and 5341 patients who had started taking valproic acid between 1993 and 2001. New valproic acid users were slightly older than lithium users (75.4 years v 73.5 years). More than three quarters of new lithium users and fewer than two thirds of valproic acid users had contact with a psychiatrist. Almost one quarter of the new lithium patients and 41% of new valproic acid users had had a diagnosis of dementia.
The number of new lithium users per year fell from 653 older adults in 1993 to 281 in 2001, whereas the number of valproic acid users rose from 183 in 1993 to 1090 in 2001. The trend was similar when we eliminated patients who had had a diagnosis of dementia (figure). The number of new valproic acid users surpassed new lithium users in 1997, with a steady decline in new lithium users and a steady increase in new valproic acid users between 1993 and 2000.