Besides alcohol, there apparently is another nasty among us that can cause serious liver damage in humans, not to mention rats. Comfrey, Symphytum officinale, is an herbaceous perennial plant commonly grown in gardens. Its leaves are sometimes eaten, but most often they are used in brewing a tea for which its devotees claim a long list of benefits. Apparently none of the claims are true; to the contrary, long term use of comfrey tea may cause severe scarring in the liver and may even cause cancer. Comfrey contains a number of pyrrolizidine alkaloids, which are the damaging agents. They are in fact the same compounds in tansy ragwort, an invasive pasture weed which has been implicated in more than a few deaths of farm animals. This one, too, destroys the liver.
Another reason for not introducing comfrey to the garden is that is usually becomes one of the most vicious weeds ever encountered. Comfrey plants produce an enormous amount of seed, all of which seems to germinate, and it also spreads around vegetatively; in fact, attempts at digging it out will usually contribute to its spread. Any broken-off piece of its substantial root system has the capacity to regenerate a brand new plant. It's a lot like bindweed or wild morning glory, and quackgrass, in this respect.
If comfrey has infested your garden it is most important that you dig it out carefully. When missed pieces of root begin to sprout, go after them immediately. You must get all root pieces out, and under no circumstances should new or old plants ever be allowed to set seed.
Folks who wish to grow an example of comfrey in an herb garden should confine its roots in a container and absolutely prevent it from setting any seed crops. A really serious infestation may require the use of herbicides, but such chemicals are likely to damage or kill desirable plants, too.
A few years ago, the WSU Extension service at Colorado State University published a list of herbs and other plants that have been documented as producing undesirable effects in humans when they are eaten or brewed into teas. Besides comfrey, a number of other rather common plants are listed with the negative consequences of their use.
Catnip and chamomile are two herbs that many gardeners grow and sometimes brew into teas. In the case of catnip, nervous system irregularities may develop; perhaps this explains the peculiar activities of cats when they encounter it. If you find yourself having a strong urge to slither around and rub against a clump of catnip in your garden, it's obviously time to get rid of it.
Chamomile frequently produces allergic rhinitis or anaphylactic shock in some people. Apparently those folks who are allergic to ragweed, asters, chrysanthemums or other members of the composite family are the ones seriously affected by exposure to chamomile tea.
A biennial garden flower, foxglove or digitalis, has escaped domestication in the Pacific Northwest and has naturalized in shady, moist locations. It is extremely poisonous and has caused serious illness and death many times. In 1979 in the Spokane area, an elderly couple died within 24 hours after drinking tea brewed from foxglove. Ironically, they thought they were brewing comfrey leaves which wouldn't have been any food for them either; if it had been comfrey, the circumstances would likely have not be so tragic.
Marigold should also not be used by people with allergies to the composite family. "Using" means ingesting, not growing, of course. All these plants are safe to grow--just don't eat them in any form.
Licorice root has been shown to cause edema, a drop in blood potassium levels, elevated blood pressure and heart failure if taken in large amounts. Even pennyroyal, which is safe as a flavoring, has been determined to be poisonous in large amounts. It can also cause spontaneous abortion in pregnant women.
Other plants that may occasionally be used in brewing teas and that should be avoided are yarrow, wormwood, St. John's Wort, senna, sassafras, lobelia, hydrangea, horsetail, goldenrod and even alfalfa.
In spite of what the popular literature has to say about herbal teas, it pays to be prudent in their use. Even though teas made from rosehips, peppermint, orange and some others are known to be safe, we can't conclude from this that all herbal teas are okay. Too much "evidence" is based on folklore, hearsay and tradition, but very little on scientific evidence.
For further information contact your local WSU Extension Office.