I Have A 12 1/2 Yr Old Female Chihuahua. I

I have a 12 1/2 yr old female chihuahua. I have been giving her Furosemide (Lasix) 50mg. 1/2 pill in morning and the other 1/2 - 12 hours later. i was wondering if i can give her...Phenobarbital (15mg) with the other pill. or can i give her 1/2 of that one also. I tried the whole pill, but it made her stumble around. Thank you for your help in this matter. Andrea Burton