PHENOBARBITAL (PB) DOSING CONFUSION

By Guardian Angel Mary Jane & Maggie May

Are you confused about your dog's dosage compared to what other epileptic dogs are taking? The reason for this confusion is the difference in weight measurement used by different pharmaceutical companies.

Phenobarbital comes in 1/8 grain, 1/4 grain, 1/2 grain, and 1 grain tablets. Phenobarbital also comes in  8, 15, 16, 30, 32, 60, 65, and 100 milligram tablets. There is a phenobarbital elixir which contains 15 mg/5 ml  (15 milligrams phenobarbital in 5 milliliters of solution).  No wonder everyone gets confused!

To simplify matters, 1 GRAIN (gr) is equal to 64.8 MILLIGRAMS (mg).  This is often rounded off to 60 or 65.

Because the milligram tablets are frequently used, the following conversion scale is helpful:

 15 mg tablet    =   1/4 grain
 30 mg tablet    =    1/2 grain
 60 mg tablet    =      1  grain

The recommended dosage of phenobarbital is 2-4 mg/kg orally every 12 hours.

To determine your dog's weight in kilograms: divide the weight in pounds by 2.2.
EXAMPLE:  A 50 lb. dog equals 22.7 kg  (50 divided by 2.2 = 22.7)
OR use our Conversion Table.

Calculating a 2 mg dose: 2 x 22.7 = 45.4 or 45 mg phenobarbital given every 12 hours.
Calculating a 3 mg dose: 3 x 22.7 = 68.1 or 68 mg phenobarbital given every 12 hours.
Calculating a 4 mg dose: 4 x 22.7 = 90.8 or 91 mg phenobarbital given every 12 hours.


References:
Veterinary Drug Handbook, Donald C. Plumb, Pharm. D., 1994.
Veterinary Emergency Medicine, "Seizures", Wayne E. Wingfield, D.V.M., M.S. and J. Michael McFarland, D.V.M., 1997.