Supplementary Material for: High Prevalence of Potential Drug-Drug Interactions for Psoriasis Patients Prescribed Methotrexate or Cyclosporine for Psoriasis: Associated Clinical and Economic Outcomes in Real-World Practice

Background: Methotrexate (MTX) and cyclosporine (CYC) may adversely interact with common medications in patients with psoriasis. Objective: Our purpose was to investigate the prevalence and outcomes of MTX/CYC polypharmacy. Methods: We evaluated rates of events that may be associated with drug-related toxicity, health care resource utilization and costs for patients with psoriasis in the Ingenix® Impact National Managed Care Database (1999–2007) who were exposed or not exposed to potential drug-drug interactions. Results: Among 4,583 (57.6%) exposed and 3,372 (42.4%) nonexposed patients, nonsteroidal anti-inflammatory drugs and antibiotics were the most common drugs with potential interactions. The exposed patients had significantly greater risks of developing renal [adjusted odds ratio (OR): 2.58; p = 0.0145], gastrointestinal (OR: 1.36; p = 0.0197) and pulmonary events (OR: 1.20; p = 0.0470), and significantly greater health care resource utilization (e.g. OR for inpatient and emergency department visits: 1.47; p < 0.0001) and costs (adjusted incremental cost: USD 1,722; p < 0.0001). Conclusions: MTX/CYC polypharmacy is prevalent in patients with psoriasis and associated with significant risks.