I keep getting poisoned!

Best way to deal with this is to make a cure spell:

Press M. (you will be prompted, "Which spell?")
Press C (you will then be presented with a menu of reagents)
Press B for Ginseng.
Press C for Garlic.
Press enter.

You just made a Cure spell! If you get poisoned, press C for "Cast" and then C again for "Cure."

These 2 reagents (Ginseng & Garlic) can be bought in shops.

There's four shops for reagents, with the ones in Moonglow and Skara Brae being the most... straightforward. The one in Paws is behind a secret wall, and the one in Buccaneer's Den requires you to dispel a field first.

Yeah but once you find the one in Paws, it's fairly easy to get to as it's not far from LBs castle and on mainland Britannia :-)