This is, of course, not to say that Medieval and Renaissance people didn't also use a wide variety of perfectly safe substances in their foods, cosmetics, and medicine. The trouble is that safe and potentially harmful things are not differentiated.
Before you use any herb-- for food, crafts, or whatever--, check its safety in a couple of modern herbals that give reliable medical information. I like Penelope Ody's Complete Medicinal Herbal, and Sarah Garland's Complete Book of Herbs and Spices; Rodale Press and Storey Publishing also produce some good herbal resources. Check the copyright date: anything from a book copyrighted before 1985 should be verified in another resource.
I love herbs and I do a lot of herb crafts and use herbal home remedies. But after 20 years of working with herbs, I still don't consider myself competent to tackle medical herbalism beyond the first-aid/home remedy stage. Like it says on the labels of over-the-counter medicine, for serious or ongoing illnesses or conditions consult a doctor. Herbal home remedies (from Grandma's 'honey and lemon' to Gypsy Cold Care brand tea) are no different. If you choose to use them, treat herbal medicines with respect. Just because it's natural doesn't mean it's safe. An old apothecary's saying is that something powerful enough to help is powerful enough to harm. An inexperienced herbalist should never mess with such remedies on his or her own-- consult a reputable medical herbalist, pharmacist or other medical professional. (Any modern book or herbalist who doesn't encourage you to also consult a physician should be considered unreliable and regarded with heavy suspicion.) Avoid things that the period herbals say are abortifacients or mind altering (psychoactive, hallucinogenic, etc.) substances; these are generally toxic. Also treat things referred to as vermifuges (treatments for human internal worms) with extreme caution: if they can kill worms, what do you think they'll do to your insides? Purgatives should not be taken internally, as they tend to imitate the effects of a really bad bout of intestinal flu, and are often outright poisonous.
Anyone can be allergic to anything. If you are making food for a group, or a fragrance or craft for someone, don't keep your ingredients a secret! Some herbs and botanicals are known to be allergens for many people, camomile and lavender among them. But there are odd allergies out there-- I know someone who gets an upset stomach from rosemary, and someone else who is deathly allergic to orange peels. If you're trying something new, be cautious yourself, too.
Scientists rightly complain that herbs and botanicals vary widely in quality and strength of active components (which cooks and fragrance crafters will confirm) from batch to batch, so the strength and potency of an herb mix can vary wildly. Essential oils, extracts, distillates and tinctures generally contain the active ingredients of herbs in much higher concentration than in the herb itself, and so can have different or more powerful effects. (I like to check out safety considerations for oils in The encyclopedia of essential oils.) Also, things that are safe for external use may not be safe for consumption.
Everything in moderation: Herbs and spices that in small quantities are pleasant can be problematic when used or taken too much or for too long a time. One cup of peppermint tea can soothe your stomach, but five or six in quick succession may have the opposite effect! Scientists continue to find that too much or too extended use of many botanicals can have negative effects. As they say about all medicines, more is not necessarily better.
Wildcrafting (picking herbs and botanicals from the wild) can be dangerous. Don't ever use or consume anything you find growing wild unless you are absolutely certain you can identify it correctly, and even then it's best to get a second opinion from an expert! Never rely on identifying something from a book. (Just because birds or animals can eat something doesn't mean it's not poisonous, either.)
Copyright 1999-2003, Jennifer A. Heise. Contact
me via email for permission to reprint: firstname.lastname@example.org
Permission is explicitly granted for limited reproduction as a printed handout for classes in schools, herb society meetings, or classes or guild meetings in the Society for Creative Anachronism (except to corporate officers and board members of the SCA, Inc.), as long as I am notified and credited and the entire handout is used.
Jadwiga's herbs homepage: http://www.lehigh.edu/jadwiga/herbs/herbs.html