A man in his 70s was transported to our hospital for acute exacerbation of chronic kidney disease. After arrival, he began having visual hallucinations, which were attributed to cognitive decline. We subsequently found that he had been taking amantadine 100 mg/day prescribed by another hospital. We thought that the visual hallucinations were caused by overdose of amantadine and discontinued the drug. Since abrupt discontinuation of antiparkinsonian drugs has a risk of neuroleptic malignant syndrome, tapering the dosage is desirable in most cases. However, the half-life of amantadine in patients with severe renal dysfunction is estimated to be 7-10 days, and thus the blood concentration of amantadine can be inferred to decrease slowly. Therefore, we chose to discontinue amantadine without tapering in the expectation of rapid improvement of the visual hallucinations, considering that the risk of neuroleptic malignant syndrome would be lower than that in patients with normal renal function who discontinued amantadine immediately. After the discontinuation of amantadine, no increase in creatine kinase level or muscle rigidity was observed, and the visual hallucinations improved on day 7 after discontinuation.

A woman in her 80s developed a feeling of abnormal sensation in her face and excessive salivation. She was diagnosed with right facial nerve paralysis and was admitted to a local hospital. On admission, serum creatinine level was 0.54mg/dL and estimated glomerular filtration rate was 79mL/min/1.73m2. She was started on oral valacyclovir at a dose of 3,000mg/day to treat the right facial nerve paralysis. However, 5 days after starting oral administration, she developed generalized fatigue, vertigo, and vomiting. Serum creatinine level rose to 4.99mg/dL with mild disturbance of consciousness, so she was transported to our hospital on suspicion of acyclovir-induced encephalopathy. We performed hemodialysis for 3 consecutive days to remove the acyclovir from the circulation, which subsequently improved all her symptoms. She was later diagnosed with allergic tubulointerstitial nephritis based on renal biopsy.After discharge from our hospital, laboratory data showed a serum creatinine level of 0.67mg/dL. We later confirmed that the serum acyclovir level before the first hemodialysis session had been very high (11.9μg/mL).