Herbs That Kill

I was wondering if there is a herb that will kill you, such as a tea or medicinal drug.

There are many herbs that when used inappropriately can cause death. Foxglove, the source of digitalis and related cardiac drugs, can easily cause death. In fact, in 1977 a couple mistakingly consumed a tea made with foxglove leaves and died. Stramonium (Datura stramonium) has caused many deaths. So has henbane (Hyoscyamus niger). Yet, these and many other dangerous plants are very useful medicinal plants when used by a trained health care giver.
In the Richters catalogue plants that are dangerous to use are marked with the code "p" (for poisonous). These should not be used for medicinal uses except by a qualified practitioner.
An excellent source of information on the toxicity of herbs is James Duke's "Handbook of Medicinal Plants", published by CRC Press in Boca Raton, Florida.