Selective contracting and patient outcomes: a case study of formulary restrictions for selective serotonin reuptake inhibitor antidepressants

Many health maintenance organizations (HMOs) have selected 1 or 2 selective serotonin reuptake inhibitors (SSRIs) as their preferred drug for treating depression. This study investigated the effect of "single-drug" formulary restrictions on the likelihood of drug therapy completion for new patients, controlling for initial SSRIs used and other factors.

Prescription drug and medical record data for 187 patients who were newly prescribed SSRIs were retrieved from a single California group practice consisting of 22 board-certified primary care physicians. The group practice contracted with 2 independent practice association-model HMOs with different SSRI formulary restrictions. A multivariate analysis of drug therapy completion was conducted and 2 sensitivity analyses were performed. Completed therapy was based on the patient having achieved 6 months of uninterrupted therapy at a minimum therapeutic dose.

Patients from the HMO with a single preferred SSRI (paroxetine) were 80% less likely to complete therapy than were patients from the HMO with 2 preferred SSRIs (fluoxetine and paroxetine) (odds ratio [OR] = 0.200, 95% confidence interval [CI] = 0.083-0.430). This formulary effect was independent of the initial drug used to treat the patient. Drug selection was also found to affect completion rates. Patients treated with sertraline were significantly less likely to complete therapy than were patients treated with fluoxetine (OR = 0.319, 95% CI = 0.105-0.968). Similar results were found for patients taking paroxetine relative to fluoxetine (OR = 0.357, 95% CI = 0.149-0.853).

These results suggest that the use of single-product formularies may have unintended consequences on patient completion rates, independent of whether or not the most effective product is selected for preferred formulary status.