Baking soda. It is not a drug but depending what is making your skin itchy there is a chemical action -- it is alkaline and will neutralize many of the irritants. It is also good for things like treating an insect bite, and in fact if you read the ingredients the after-bite products on the market are mostly just baking soda sold at a higher price. For a similar reason lemon juice may help. Some people mix baking soda and lemon juice together as well.
You can make a paste out of it and rub it on your skin, or mix it in water and soak in it.
It's more likely to help with an external irritant (something that actually irritated your skin by contact, like a bite) than with an internal histamine reaction to a food allergy, etc., but you can always try anyway, it won't hurt you.