Background: Our previous placebo-controlled, prospective, double-blind study demonstrated that a new opioid ĸ-receptor agonist, nalfurafine hydrochloride, effectively reduced treatment-resistant pruritus in 337 hemodialysis patients. Thus, we designed this study to evaluate prospectively the efficacy, safety, addiction liability, and pharmacokinetics of nalfurafine given orally for 1 year. Methods: This open-label study examined the effects and adverse drug reactions (ADRs) of 52-week oral administration of nalfurafine hydrochloride (5 µg/day) in 211 hemodialysis patients with a treatment-resistant itch. Results: Of 211 patients, 145 completed the study as scheduled. The mean pruritus value assessed by the visual analogue scale was 75.2 mm during the pre-observation period, which decreased significantly to 50.9 and 30.9 mm in week 2 and 52, respectively, indicating a long-lasting efficacy. ADRs occurred in 103 patients (48.8%). Frequent ADRs were insomnia (sleep disturbance, 19.4%), constipation (7.1%) and increased blood prolactin (3.3%), similar to previous reports. Regarding addiction liability, it appeared unlikely that nalfurafine hydrochloride was abused. After the start of treatment, plasma drug levels reached a steady state in week 2 with no apparent tendency of systemic accumulation. Conclusions: Nalfurafine hydrochloride, orally administered at 5 µg/day for 52 weeks to hemodialysis patients, produced a long-term suppression of pruritus without significant safety problems.

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