Prevacid can cause allergic reactions.

A friend of mine's husband had a huge reaction out of the blue from Prevacid! His reaction was severe and lasted two weeks from one capsule!

You might discuss changing to another drug.

There are some cross sensitivities of Prevacid with penicillin.
I don't recall if other antibiotics were included, however.

What is the other stomach med you are using?

The two best antihistamines for skin itching are Atarax (RX) and Benadryl (OTC). Zyrtec is a cousin of Atarax and may help. I don't think Allegra is very effective for itching. It is a mild antihistamine.

Periactin is an odd duck. It is used to encourage weight gain.
Because of the side effects and also the antiserotinergic effects, it is
not commonly used as an antihistamine, since some others are just as
potent.
http://en.wikipedia.org/wiki/Cyproheptadine

It is often not found in pharmacies, so if you do get an RX for it, it may
have to be special ordered for you.
I don't believe Merck makes the brand anymore. I only see generics listed
at the FDA.gov site.

Zyrtec and its generic are OTC now. I have found that sometimes you can
get good effects with 5mg or 1/2 tablet even.

Avoid Claritin-- it does nothing IMO.