Okay. I can respect my Nana's religious beliefs, and politely defend my own in a discussion. That's fine, and I've no problem with what she believes. I will not knock her faith in that circumstance.
I will do everything in my power to knock her faith in 'the good earth' if the upshot of it is that she genuinely and steadfastly believes that anything 'herbal' is automatically good for you, and you can eat as much of it, and in as many combinations, as you want. If it's 'natural' and comes from the earth, it is good for you no matter what you do with it.
Except, you know, no. No, it really fucking isn't, Nana. Opium is natural (with some processing). Quite a few viciously deadly poisons are 100% natural. Medicinal herbs, when taken in the wrong combinations or at the wrong doses, can be really fucking bad for you. You can't, you really can't, take whatever you feel like, and trust it won't kill you just because it's 'natural'. And sorry, but I will attempt, strenuously, to destroy your faith in 'the good earth', if that's what your faith leads you to. For heaven's sake, where do you think humanity got most of the basis for its poisons from. If you are taking it for it to have an effect, that means it is changing something in your body, and if you do it wrong, then it changes your body in the wrong way.
(I am irresistably reminded that Granny Weatherwax once drank absinthe because it was misrepresented to her by Nanny as 'green and herbal, and therefore good for you'. But Granny is badass, and a witch, and can get away with shit like that. My Nana can't.)
For the love of sanity, woman, at least talk to the herbalist/chemist/health shop selling you the herbs, and do things like, you know, tell them what else you're taking, and listen when they tell you how much to take, and don't mix things they tell you not to mix just because some book tells you something is better for such-an-ailment than what they gave you. I don't care how trustworthy the goddamn book is. The book doesn't know what else you're taking, or your personal medical conditions, or a whole host of other things that the person you talk to, however untrustworthy, at least has a better chance of being aware of.
*rubs face* You can't just put any old plant in your mouth, you know? For the love of Someone, haven't you told your kids that enough times? Why do you suddenly decide it's not true now?