These cautions are for the herbal and essential oil form of the plant. If used homeopathically, the same evaluation does not apply.
Aconite: very toxic. It is poisonous and should not be used without medical advice. There is no antidote. One mg. can kill a horse. It can be absorbed through the skin.
Aloe: fresh juice helps heal minor wounds and burns. Taken internally it is a violent purge if the the whole leaf is used. The inner leaf latex is the purgative. Juice made from just the gel is safe..
American hellbore (black and green): Internally violently narcotic. Symptoms of hellbore poisoning include salivation, nausea, vomiting, colic, diarrhea, weak heartbeat, vertigo, ringing ears, disturbed vision, coronary arrest. Green hellebore is a cardiac depressant...black hellbore is a cardiac stimulant. Applied locally, the fresh root is an irritant.
Angelica: Is carcinogenic and mutagenic, especially in the presence of sunlight.
Arnica: Taken internally is an active irritant which produces toxic gastroenteritis, nervous disturbances, pulse rate changes, muscular weakness, increased blood pressure. May be used externally if there is no danger of ingestion.
Autumn crocus: Lowers sexual excitement (probably because ite causes nausea, intestinal pain, diarrhea, vomiting, and sometimes death.) Lethal dose is about 65 mg in humans, but 7 mg has been proven to kill.
Bayberry: Causes cancer when injected. Bayberry wax is a sensitizing irritant.
Bloodroot: Internally causes burning in stomach, intense thirst, paralysis, vomiting, faintness, vertigo, vision problems.
Blue cohosh: Toxic to the heart muscle and may harm intestines. Seeds are poisonous. Powder is strongly irritating to mucous membranes.
Broom: Slows and weakens heartbeat. Toxicity symptoms similar to nicotine poisoning: circulatory collapse, irregular heart beat, nausea, diarrhea, vertigo, headache, paralysis of respiratory and motor centers, convulsions, death.
Coltsfoot: Is used for coughs but contains alkaloids that cause liver cancer.
Comfrey: Contains allantoin and the alkaloids consolidine and symphytocynoglossine. Concentrrations as little as 0.5% of the diet have caused cancer in rats.
Deadly nightshade: Contains alkaloids hyoscyamine, atropine, and hyoscine. Properties are sedative, narcotic, and act on the central nervous system. Small doses stimulate and large doses paralyze. Belladonna poisoning manifests within 15 minutes of ingestion.
Eyebright: 10-60 drops of the tincture can induce mental confusion and violent pressure in the eyes with lacrimation, itch, redness and swellings of the margins of the lids, dim vision, photophobia, weakness, sneezing nausea, insomnia, polyuria. (don't use a tincture, use a tea)
Foxglove: Symptoms of digitalis poisoning include nausea, diarrhea, stomachache, headache, irregular heartbeat and pulse, tremors, convulsions, and death.
Goldenseal: (Yellow Root) Extremely bitter herb that has little effect below 'near toxic' doses. Overdoses even externally can cause ulceration of any surface it may touch. (Another source says this drug is nontoxic, nonirritating, antiseptic, both heals and soothes)
Hemlock: Symptoms of lethal dose: paralysis of tongue, head pressure, giddiness, nausea, vomiting, diarrhea, central paralysis - frist feet and legs, the buttocks, arms, them swallowing and speech. Death ensue through central respiratory paralysis. Socrates suicided on this herb.
Jimsonweed: People have died trying to get high on Datua. Hallucinations, irrigular heartbeat, elevated blood pressure, delirium, dilatation of pupils and convulsions are some of the symptoms. Overdose is usually fatal.
Juniper: Not toxic in tiny doses used to flavor gin. Small repeated doses may cause convulsions and kidney damage. Other internal symptoms are pain in or near kidney, strong diuresis, albumin and blood in kidneys, purplish urine, accelerated heartbeat and blood pressure.
Licorice: Alleviates peptic ulcers but causes edema (swelling of tissues) and increases blood pressure. Large doses over extended periods of time are toxic.
Life root: Eases pain of childbirth but can damage liver because of the presence of the alkaloid senecionine.
Lobelia: It's deadly. In normal doses produces dilation of bronchioles and increased respiration but overdoses cause respiratory depression, sweating, rapid heartbeat, low blood pressure, coma, and death.
Mayapple: Mandrake is toxic regardless of whether it is from Europe or America. The European version damages the heart whereas the American version damages the intestines. Amerindians once used the young shoots for suicide. (Illustrated in the Disney movie: "The Light in the Forest")
Mistletoe: Ingestion of berries and leaves cause disruption of heartbeat and eventual death.
Pennyroyal: The essential oils consist of 85-92% pulegone and are toxic causing severe liver damage in relatively small amounts. The FDA recognizes pennyroyal TEA as GRAS (generally recognized as safe) if used as a beverage.
Pokeweed: Hs no known good effect. Children who have eaten the berries have died. Narcotic effects have been observed. Is used internally in chronic rheumatism but is not therapeutically useful. It acts as an emetic and cathartic because it is extremely toxic.
Rue: Used as an insect repellent, causes rash, is an acro-narcotic poison if taken internally, is an irritant externally from handling the foliage, flowers or fruit. The oil is a dangerous abortifacient (causes abortions).
St. John's wort: Causes shedding of wool in sheep, swelling of face, skin irritation, loss of appetite, photosensitization. Is also a tranquilizing action in humans. Don’t use with depressants.
Sassafras: Less than a teaspoon of the oil causes degeneration of the heart, liver, and kidneys. Safrole, the chemical constituent in Sassafras is forbidden by the FDA from use as flavors or food additives because it is a known carcinogenic.
Sweet flag: Oil of calamus causes heart and liver damage and cancer in animal studies.
Tansy: Oil is quite toxic and should be used only with extreme caution. Ten drops can be lethal. When this report was written, Tansy was prohibited for botanical dealers and could be sold as a dried herb by mail. Symptoms of internal tansy poisoning include rapid and feeble pulse, sever gastritis, violent spasms, and convulsions.
Wormwood: Is a central nervous system depressant. Acquired its reputation as a subtle poison when it became the principal flavoring ingredient in a 136 proof alcoholic beverage called absinth. Practically every civilized country in the world banned the preparation or consumption of absinthe.. Habitual use or large doses cause convulsions, insomnia, nausea, nightmare, restlessness, tremors, and vertigo.
Yohimbe: Used as a aphrodisiac, it causes a tingling sensation in the genitals. It dilates the blood vessels of the skin and mucous membranes and lowers blood pressure.