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Reply To: Phenobarbital toxicity

Emma Holt

Hi Areti,

I haven’t seen thrombocytopaenia associated to phenobarb, but I did have a case of phenobarbital induced neutropenia a few years ago. The dog improved on both haematology and clinical signs after stopping the medication initially, but sadly relapsed (despite not re-starting phenobarb) and was diagnosed with immune-mediated neutropenia on a bone marrow biopsy and steroids were started, but she didn’t do very well in the long run.

I found this article which may be of use, I’ve just read the abstract, but it looks like median resolution of haematological abnormalities was 17 days post phenobarb withdrawal.

Phenobarbitone-induced haematological abnormalities in idiopathic epileptic dogs: prevalence, risk factors, clinical presentation and outcome
E Bersan 1, H A Volk 2, C Ros 3, L De Risio 3

The aim of this retrospective study was to assess prevalence, risk factors, clinical presentation and outcome of phenobarbitone induced haematological abnormalities (PBIHA) in dogs. The medical records of two veterinary referral institutions were searched for dogs diagnosed with idiopathic epilepsy and treated with PB as monotherapy or polytherapy between March 2003 and September 2010. Sixteen dogs had PBIHA; the median age at diagnosis was 69.5 months. Phenobarbitone was administered at a median dose of 3 mg/kg twice a day for a median period of 100.5 days and the median serum phenobarbitone level was 19 μg/ml. Two dogs had neutropenia, three had anaemia and thrombocytopenia, two had anaemia and neutropenia; the remaining nine had pancytopenia. All dogs were referred for non-specific clinical signs. Phenobarbitone was discontinued after diagnosis, and the median time to resolution of PBIHA was 17 days. The prevalence and risk factors for PBIHA were evaluated from a questionnaire survey of referring practices to obtain more detailed follow-up on cases diagnosed with idiopathic epilepsy. The prevalence rate of PBIHA was 4.2%, and the condition occurred in dogs treated with standard therapeutic doses often within the first three months after starting treatment. Serial haematological evaluations should be therefore considered from the beginning of phenobarbitone therapy to allow early diagnosis and treatment of PBIHA.