NAPLEX Question of the Week: PhenoBark Prescription?

This week's question details a common medication prescribed to dogs.
NAPLEX Question of the Week: PhenoBark Prescription?

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KS brings a new prescription into your community pharmacy for his 4-year-old dog, Champ. Champ has several antiseizure medications on his profile. After talking with KS, he informs you that Champ’s seizure frequency has increased over the past the month and the veterinarian has increased the dosage of one of his medications. The new prescription is for Phenobarbital 1.5 grain: 1 tablet PO BID. 

Medications on profile (filled on time over past 3 months):

Phenobarbital 64.8 mg: 2 tablets PO BID #60

Keppra XR 500 mg: 5 tablets PO BID #300

Based on the prescription, how should Champ’s medication be filled? (MC)

A. Fill the prescription as Phenobarbital 64.8 mg 2 tablets PO BID

B. Fill the prescription as Phenobarbital 97.2 mg: 1 tablet PO BID

C. Call the veterinarian’s office to confirm prescription

D. Fill the prescription as Phenobarbital 70 mg: 3 tablets PO BID


Brands/Generics covered: Keppra (levetiracetam), Luminal (phenobarbital) 


While the “grains” unit is not commonly used, there are various professions that pharmacists will encounter that still incorporate it. Pharmacists in the community setting will be exposed to numerous types of patients, including veterinarian patients. Common veterinary prescriptions seen in the community setting may include anti-anxiety, pain, allergy, and seizure medications.

Epilepsy is a neurological disorder that causes recurring, unprovoked seizures. Seizures are caused by imbalances of inhibitory and excitatory neurotransmitters that result in surges of excessive electrical signals in your brain. Epilepsy and seizures can be managed with anticonvulsants, including barbiturates.  Dose adjustments should be monitored carefully because subtherapeutic dosages can precipitate seizures. Therefore, it is important as pharmacists to compare prescribed dosages to medication profiles and check with patients to ensure they receive the right medication at the right dose.

1 grain = 64.8 mg.

Conversion of new prescription to milligrams: 1.5 grain x (64.8 mg/ 1 grain) = 97.2 mg.

Phenobarbital dosage of old prescriptions: 64.8 mg/tablet x 2 tablets = 129.6 mg.

Using the conversion above, the new prescription is for Phenobarbital: 97.2 mg by mouth twice a day. Based on the patient’s profile and owner history, Champ was experiencing breakthrough seizures while receiving Phenobarbital 129.6 mg twice a day. Therefore, the dosage of the new prescription is actually a decrease in dosage that could lead to worsening seizure control.

Correct Answer: C

Answer A is incorrect: This is the wrong conversion based on the new prescription. Additionally, filling for the same strength and instructions as previously filled could result in subtherapeutic phenobarbital levels needed for Champ’s seizure control.

Answer B is incorrect: While this is the correct conversion of grains to milligrams for the new prescription, this dosage would be subtherapeutic based on previous fill history and patient history of breakthrough seizures. With the given information, the prescribing veterinarian should be consulted to confirm the prescription.

Answer C is correct: Converting the new prescription from grains to milligrams yields a result of 97.2 mg. Based on the instructions of the new prescription and the patient’s medication profile, the new prescription would be a decrease in dosage. The prescribing veterinarian should be consulted to confirm and/or adjust the prescription to ensure adequate seizure control.

Answer D is incorrect: This is the wrong conversion based on the new prescription. While this would be an increase in dosage, the prescribing veterinarian should be consulted prior to adjusting the dosage. Furthermore, Phenobarbital is not available in a 70 mg tablet.

NAPLEX Competencies Covered:

Area 1 (Obtain, Interpret, or Assess Data, Medical, or Patient Information), 1.2 From patients, 1.4 Medical Records, 1.5 Signs or symptoms of medical conditions. Area 4 (Perform Calculations), 4.4 Dose Conversions; Area 6 (Develop or Manage Practice or Medication-Use Systems to Ensure Safety and Quality), 6.1 Interdisciplinary practice, collaborative practice, or expanded practice responsibilities

Have a great weekend everyone!

Dr. B

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