Hmmm. Does the vet you use approve of any mental health meds. Some pets need things like prozac and other antidepressants. Something along those lines may be the better route. Though, like in humans, it does take a while to show its effect.

My family had a cat (16 years old) with dementia, hyperthyroidism and later cancer as well. What ended up calming her was Valium/Diazepam. First two where quite good to handle with meds and she ended up gaining weight and less vocal/yelling. Eventually she ended up having cancer and they decided it was ready to let her go. But in the mean time she had a good and even beter quality of live with the Valium.