Will Taking Rimadyl Be Safe For Osteoarthritis?

My Border Collie was placed on Rimadyl 75mg q 12 hours for osteoarthritis of his hip and became very aggressive and confused if woken up during the night with family members. He will be 10 years old in May, has no prior medical history except for his arthritis, he is a pure blood Border Collie. He had been on the intial two weeks of Rimadyl at above dose when he be became more aggressive and confused when woken out of a sleep mode. The Rimadyl help his greatly with gait and pain,l but cut him back to 1/2 tablet once aday with no aggression or confusion noted. I notice
Rimadyl half life is appx. 8 hours, will this be enough to cover his arthritis pain for 24 hours, or is there another medication to consult with his veterinarian which cause less confusion and aggression. He does not display any other side effects being on this medication.

Concern Owner