Following are some very insightful comments on herb-to-herb and herb drug contraindications submitted to the forum by herbalist, Andrew Bentley. I consider them to be very insightful and of special relevance that with Mr. Bentley's permission, I am reprinting them in the articles section. Keep in mind this is not a finished article but a series of online responses. Michael Tierra
Onions contain volatile chemicals of the isothiocyanate class. Isothiocyanates caused the Union Carbide chemical disaster in Bhopal, India. So, one can scientifically conclude that cutting up an onion will kill tens of thousands of people.
Are you saying your experience differs? No matter. Your experience was not in the form of a controlled study, and has not been published as a case study in a peer-reviewed journal. It's merely anecdotal. Better stay away from cabbage and watercress too. Scientific evidence shows that these plants are deadly.
Sassafras is a tree. There are two types, white and red (S. albidum et S. officinalis). Most folks will tell you there isn't any more red, because they don't want you to take theirs. The roots and the bark of both types are used as a blood tonic. Virgil J. Vogel (in his book American Indian Medicine) says that when Europeans first discovered Sassafras, it caused a bigger fad than any herb before or since, including tobacco.
Sassafras is sort of a general restorative, used most often after the winter to purify the blood, keep people from getting sick when the weather changes, prevent seasonal allergies, and so on. It is said to work on the liver, which Appalachian herbalists will tend to say about plants which have three-part leaves (hepatica, clovers, ragweed). As a specific medicine, Sassafras is used in cases of viral, fungal, bacterial infections and also cancers, and occasionally things like arthritis, which can be helped by purifying the blood.
Within Sassafras there is a chemical called safrole. Safrole is a very common plant chemical, found in Mace, Nutmeg, basil, black pepper, Rosemary, Dill, Black tea, Dang Gui, Tamarind, Cinnamon, witch hazel, Asian wild ginger, and many other plants.
Someone did a study demonstrating that this substance was carcinogenic in rats. Dr. James Duke reports that even if this same carcinogenicity were applicable to humans, a cup of sassafras tea would be 1/14 as carcinogenic as a cup of beer. Sassafras has been banned by the FDA as a commercial product, unless it has had the safrole removed. Therefore, the smack has been put down on what could be and once was an important part of the economy in some of the nation's poorest areas.
So, sassafras is a bit carcinogenic to rats. More specifically, a metabolite of safrole is carcinogenic to rats. Humans don't produce this same metabolite. At any rate, this comes as no surprise to anyone who knows about sassafras. We've used it for centuries to deter rodents. Boxes made of sassafras have been used to store furs to keep rats from eating them. Sassafras leaves have been and still are placed in the corners of houses to repel vermin. We could have told you it wasn't good for rodents. If sassafras were edible to rodents, there wouldn't be any. Why? Because sassafras grows where beavers (which are large rodents who eat tree bark) live, and they would probably enjoy this sweet tasting tree to its very extinction.
Furthermore, tradition states that, in humans at least, sassafras is useful for preventing the spread of cancer. Does phyto-chemical analysis back this up? Sure... Alpha-pinene, anethole, eugenole, myristicin, and, yes, safrole are all chemicals contained in the plant which have this property. But it's not calculus class, or even Organic Chemistry 101. You can't just add up the +'s and take away the -'s and get the answer. It's real life, and must be dealt with as the infinitely complex system that it is.
Comfrey is a similar situation, contains fewer hepatotoxic chemicals than an organically grown carrot (according to the CRC handbook of Phytochemical constituents of GRAS herbs and other economic plants, James A. Duke, 1992), and is therefore one of the most deadly herbs out there, to hear the media and some scientists tell of it. Seriously, if these people want to talk about a dangerous herb, why not focus on something like raw Aconite? I mean, that stuff kills you.
Comfrey is sometimes eaten as a vegetable by Appalachians, Irish people, Russians, and at least at the time M. Grieve's "Modern Herbal" was written, some people in Britain.
There are case studies of people who had eaten comfrey or drunk the tea getting liver diseases. Maybe some people can't handle it. I know I've almost died from eating tomatoes (just the fruit), when I was very young. That's a fruit many people eat with no problem, but something about it didn't agree with my system. At the same time, I've eaten a fresh raw aconite root with no problem.
So, not everyone's body will respond to herbs in the same way, and with something as commonly used as comfrey, I could see the occasional weird thing cropping up now and then. At the same time, the studies generally pay no attention to other possible factors such as lifestyle (drinking, possible liver damage beforehand, medications the person was on), or on how the plant was grown and processed. So I don't feel like they were very good case studies.
By and large the rules for combining herbs and drugs are a lot like the rules for combining herbs with other herbs. The main difference is that there are not a lot of people who know much about drugs and know much about herbs. Usually the worst thing that's going to happen is that the herb will counteract the drug. I concur that adverse reactions are not common. In fact, they're surprisingly uncommon, considering the amount of people who use herbs and the frequency with which herbs marketed for conditions where they are not indicated. I still feel that all clinicians use either herbs or drugs in their practice should be familiar with herb drug interactions. People will want to know about them.
There is a book, Herb Contraindications and Drug Interactions by Francis Brinker, which provides interesting and somewhat useful information on the subject. HRF also has an information packet on the subject, which I didn't find to be especially revealing. It might be a good intro for the lay person, though.
Blood thinners are one class of herbs that make for a perennial topic of concern. Coumarin-containing anticoagulants, which slow the formation of clots, are not as pronounced in their effects as herbs which inhibit PAF (platelet activating factor), such as Wintergreen, willow, and violet. Because herbs usually do not provide high levels of PAF-inhibiting activity, this effect is not necessarily noticeable, but one should at least be aware of the possibility.
Another common class of interactions is that of Alkaloid drugs and acidic herbs or herbal preparations. Herbs which contain large amounts of tannins (Oak bark, black tea, raspberry, etc.), salicins (Willow bark, violet roots, spirea etc.), formic acid (conifers, croton), rosmarinic acid ( Heal-all, Marjoram, rosemary, etc.), and a host of organic acids (the sour taste) or anything made with an acidic menstruum such as vinegar, won't combine so well with alkaloid drugs (Too numerous and varied to represent). Now, the reason for this would be that the alkaloids and acids would interact to create other substances, likely to be less bioactive. For this to happen, they have to physically contact one another. So this sort of an interaction is likely to manifest if the drugs and the herbs are in the stomach together, and the drug is not enteric coated. Drinking black tea with your Phenobarbital, for example. Spacing the times that they were taken as widely as possible would help to work around such an interaction (although, of course, nothing in this business is 100% certain). Unfortunately, most of what has been published about herb-drug interactions is speculative. Quite often, it's ill-informed speculation. Frequently, it's sensationalized, even the stuff that's in peer-reviewed journals. One of my favorite examples of this is the myth that Kava has caused Coma in a patient taking alprazolam. Now, the origin of this Urban Legend was when a man who had been taking these two substances came to the hospital because he was experiencing confusion and disorientation, a well-documented side effects of the anti anxiety drug Alprazolam. His physician described the confusion as a "semi-comatose state". He was never in a coma, just dazed. But this worked its way into the medical literature as a letter titled "Coma from the Health Food Store" in the Annals of Internal Medicine, Dec. 1996. Later, referring to the events of this letter (perhaps without having read it?), an article in the Archives of Internal Medicine (Nov. 1998) stated that "Kava when used with alprazolam has resulted in coma". But there was no coma, nor anything substantial to suggest that the Kava played a part in the adverse reaction at all. It's a typical Urban Legend scenario. When people are frightened they blow things out of proportion, and they tell as many people as possible.
So be cautious when using herbs with drugs, make every effort to work in synergy with whomever is prescribing the drugs to reach the greatest possible understanding, and read material on the subject with a critical eye.