Objective To examine the impact of cost‐sharing increases on continuity of specialty drug use in Medicare beneficiaries with multiple sclerosis (MS) or rheumatoid arthritis (RA). Data Sources/Study Setting Five percent Medicare claims data (2007–2010). Study Design Quasi‐experimental study examining changes in specialty drug use among a group of Medicare Part D beneficiaries without low‐income subsidies (non‐LIS) as they transitioned from a 5 percent cost‐sharing preperiod to a ≥25 percent cost‐sharing postperiod, as compared to changes among a disease‐matched contemporaneous control group of patients eligible for full low‐income subsidies (LIS), who faced minor cost sharing (≤$6.30 copayment) in both the pre‐ and postperiods. Data Collection/Extraction Methods Key variables were extracted from Medicare data. Principal Findings Relative to the LIS group, the non‐LIS group had a greater increase in incidence of 30‐day continuous gaps in any Part D treatment from the lower cost‐sharing period to the higher cost‐sharing period (MS, absolute increase = 10.1 percent, OR = 1.61, 95% CI 1.19–2.17; RA, absolute increase = 21.9 percent, OR = 2.75, 95% CI 2.15–3.51). The increase in Part D treatment gaps was not offset by increased Part B specialty drug use. Conclusions Cost‐sharing increases due to specialty tier‐level cost sharing were associated with interruptions in MS and RA specialty drug treatments.