I have a couple of certificates in 'herb magik', so feel free to ask away or request any further info
Belladonna (deadly nightshade)
Byrony (european white, white, and black)
Calotropis (cherry Laurel)
Dropwort, Hemlock water
Hellbore (black, false,green, white hemlock)
Nightshade, deadly, nux vomica
Wake Robin, American
Poisonous ingredients were probably added in the dark side of the trade in ancient times to harm, banish or ward off. We, of course, don't want to do this in this day and age! What about Tobacco? It is a great substite for those plants such as belldonna, nightshade, hemlock, cowbane or hellebor, it is still poisonous but you aren't going to die from inhaling a bit of tobacco leaf. Nettle is another one you can use, it isn't poisonous but you can get a 'sting' from it if you touch it. Also, you will find that a spell that ask for a poisonous herb is not for consuming, so don't panic about finding something 'edible' for your substitution!
Magikal substitutions are not dangerous, nor do they make your spells/recipes powerless. As long as you think about their uses and follow these basic guidelines - as well as your intuition - then you will have no problems at all.
So, think about the ingredient you are wanting to use, but don't have. It doesn't mean you simply put away a recipe because the ingredient is hard to get or you simply don't have it laying around - or it is a poisonuous herb. Think of the herbs properties, its energies and elements - and its scent. You should always keep woods with woods, flowers with flowers, gums with gums, oil with oils and never think for one minute that something else can not be used instead of.