What Happens When Dogs Overdose on Phenobarbital?

Two cases were presented in the Journal of Veterinary Emergency Critical Care this week in which phenobarbital was ingested at toxic doses. This paper served as a reminder to me about the risks of phenobarbital use, so I thought I'd pass those reminders along to you!

1) Phenobarbital is a potent barbiturate in the same category as pentobarbital (an often used euthanasia solution) therefore dose calculations should be taken seriously! Consider having a check system in place for techs and doctors to ensure the correct dosage is written on treatment forms.

2) Serum phenobarbital concentrations were incredibly high (see below) yet the patients were alive on arrival. This serves as a reminder that although we don't want dogs (or cats) to regularly have serum phenobarbital concentrations above 45 ug/ml, you have a long way to go to cause a fatality at serum concentrations in the 40s or 50s. Don't panic, please don't acutely discontinue the drug, but do decrease the dose!
** If you want to read this paper in depth please see the reference at the bottom.

Case 1:
9 year old MC Poodle presented to a vet for routine cystotomy. The dog had been receiving phenobarbital for about 1 year for seizures. After surgery he received a dosage of 27.2 mg/kg PO q12hr for three consecutive doses. After the 3rd dose, the dog was described as being in a coma with reduced brainstem reflexes. The serum phenobarbital concentration was 95.5 ug/ml.

Case 2:
3.5 year old FS Beagle self overdosed after ingesting 200 mg/kg phenobarbital 3 hours prior to presentation to the vet. She was also comatose with bilateral miosis, absent palpebral, menace and reduced gag reflex. The serum phenobarbital concentration was 98.2 ug/ml and spiked up to 119 ug/ml before treatment was started.

Key Points:
The authors successfully treated both cases with hemodialysis. The serum phenobarbital concentration was 12.6 ug/ml at the end of 120 minutes of dialysis for case 1; the serum phenobarbital concentration was 26.6 ug/ml at the end of 180 minutes of dialysis for case 2. These are the first two described cases of phenobarbital intoxication treated by hemodialysis!

Kudos to the authors for writing this up and reminding us that these drugs are wonderful when taken properly and toxic if not taken as prescribed!

** Basile JK, Vigani A.Treatment of phenobarbital intoxication using hemodialysis in 2 dogs. JVECC January 2020. Online https://doi.org/10.1111/vec.12908