Did you use a generic brand or the actual Periactin?  Dispite what you may
have heard, generic formulations certainly do vary.  The worst effect I've
seen were only mild and transient sedative effects and occasional
paradoxical agitated behavior in a few cats- and I've treated many cats with
cyproheptadine.

Were your cats taking any other medications?  Some other drugs can produce a
synergistic sedative effect on the CNS when taken with cyproheptadine.
Also, some medications can cause partial or complete anorexia that can
override the appetite- stimulating effects of cyproheptadine.
Chemotherapeutic drugs almost always cause anorexia and frequently alter
smell and taste perceptions- which can utterly wipe out appetite.  IIRC,
Zipper had cancer and was put on chemotherapy after the surgery to remove
the mass.

Cats are usually given appetite stimulants because an underlying disease is
causing anorexia.  Cyproheptadine isn't a panacea and it won't 'cure' the
cause of the anorexia- it will only stimulate a cat's appetite unless the
underlying disease is serious enough to override the appetite- stimulating
effects, (e.g., cancer cachexia, hepatic lipidosis or other liver diseases).

I don't think the sedative effects were as profound as they may have seemed
to you. OTOH, they could have been the result of a combination of the
effects of the underlying disease and/or other medications.  As I said, I've
treated many cats with cyproheptadine and I haven't seen the profound
effects you've described when the drug is administered at the normally
prescribed doses.

Anyway, my advice was simply to start out

Since cyproheptadine is a prescription drug, I think people should follow
their vet's prescribing instructions rather than arbitrarily changing the
dose based on an exceptional case of side effects rather than the norm.
Cyproheptadine is prescribed to most cats to jump start their appetite
because they haven't been eating for awhile.  Thus, I think its wiser to
begin therapy with the normal dose- to get the cat eating again and taper
the dose if the sedative effects seem unpleasant.  The consequences of
anorexia in cats are more severe than mild sedation.  It boils down to the
risk-to-benefit ratio. In the case of anorexia, the benefits of
cyproheptadine greatly outweigh the risks.

I'm sure it is perfectly safe

As I said, cyproheptadine will only stimulate a cat's appetite unless the
underlying disease is serious enough to override the appetite- stimulating
effects- it won't 'cure' the cause of the anorexia.

4 mg b.i.d. is at the high-end of the dosing schedule and could produce a
more pronounced sedative effect.

Buddy had the same

Dexamethasone is about 10x more potent than prednisone.  What type of liver
disease does Buddy have that requires such a potent drug?

So your vet agreed with my vet, at least as far as which antihistamine to
use. Interesting. Whether it works or not probably depends on the cat.
Even with humans the same medicine does not work the same on
different people. Let us know how it works out.

FWIW, I asked my vet what to give my cat when she kept scratching
until she had little raw spots on her ears. That was the second time he
prescribed Chlortrimeton. The first time, she had congestion from
allergies that had made her nose sore. (She was uncharacteristically
avoiding being touched on her face.) She has EGC and asthma and
tends to overgroom.

My vets are Drs. Sullivan, Price, and Driscoll at Six forks Animal
Hospital in Raleigh, NC. (They have a web site.) I like them because
they are three different generations, all graduates from the NC State
Vet School. They are pretty careful, clearly love cats, and do a lot of
gratis work for the community.