Although I am not a veterinarian I do have access to some excellent books. Although tranquillisers are useful for calming an injured or frightened cat or for relieving anxiety in general or for specific reasons such as transportation, there are side effects.
Tranquilizers for cats block cortical inhibitory impulses which may be evident in that the cat no longer uses the litter tray. Or the cat might bite and scratch at the slightest provocation.
As a result it can be difficult to carry out behavior modification with a cat on tranquillisers.
One tranquilizer used for cats and people is Diazepam, aka Valium. A side effect is that it has been shown to cause serious liver problems in some cats and it should not be used routinely.
Frequent liver enzyme evaluations should be conducted on cats who are on Valium. Valium is 55-75% effective in stopping inappropriate elimination in cats but as soon as the drug treatment stops the inappropriate elimination resumes.
It is therefore inappropriate for long term use on felines.
Wedgewood Pharmacy online (Dr Barbara Forney) refer to Acepromazine maleate (‘Ace’) as a commonly used tranquilizer in the US for cats, dogs and horses. In fact this tranquilizer is heavily referred to when searching for information on feline tranquillisers. It appears to be a major, if not the major tranquilizer for cats in 2019.
However, it can produce the opposite effect to that desired: the cat becomes aggressive or excited.
And Acepromazine is said to be unsuitable for cats to calm them down when fireworks cause anxiety. This is because it does not help to reduce fear and anxiety under these circumstances. It is worse than that; it stops the cat moving and exhibiting signs of anxiety and fear. It acts as a chemical straightjacket. It can also increase a cat’s sensitivity to noises (source: Preventative Vet online).
Acepromazine can also cause hypotension, a decreased respiratory rate and bradycardia. Hypotension is low blood pressure. Bradycardia is a heart rate that is too slow. Other side effects are: collapse, absent pulse, pale gums and unconsciousness. It can be fatal but this is rare. It causes a decrease in the ratio of red blood cells to the total volume of blood (hematocrit).
You should ask your vet for more information on this feline tranquilizer before consenting to its use.