Hello, my name is***** and I have over 20 years of experience as a veterinarian. I understand that you are concerned because your fellow Owen received an extra dose of his phenobarbital medication tonight because he has had a couple seizures over the last 24 hours.
The usual recommended dose of phenobarbital is 1mg to 2mg per pound of body weight every 12 hours, but the dose can go as high as 4mg per pound of body weight twice daily. You don't say how much he weighs.
If he normally takes 60mgs and he received an extra dose (double his usual dose) then he received 120mgs tonight. As long as he weighs more than 30 pounds then he has taken 4mgs or less per pound of body weight, which is within a normal, although higher than usual, dosing range.
I would not expect any problems with that dose other than possible mild sedation. Dogs that take phenobarbital regularly metabolize it well, and he should have no trouble with this dose, especially if he has had recent blood tests that showed his dosing level to be within range.
I think he will be just fine, though he may be quite sleepy tonight. I do recommend that he be kept away from stairs and watch him closely so he doesn't stumble and hurt himself. I would let him sleep and he should be back to himself in about 12-24 hours.
I would plan on giving him the next dose of phenobarbital as planned, unless he still seems excessively sleepy.
Please let me know if you have any further questions.