The term “therapeutic margin” refers to the gap – or “margin” – between the therapeutic (beneficial) dose and the toxic dose of a given substance. The wider this gap, the higher a therapeutic margin the substance is said to have, and the safer it is to use. Herbalists typically restrict their practices to herbs with high or moderate therapeutic margins under ordinary circumstances, and only resort to the use of low-therapeutic-margin herbs in rare situations. (Herbs with low therapeutic margins are more commonly used as the basis for pharmaceutical medicines.)

When an herb has a high (or “wide,” or “large”) therapeutic margin, a good deal of the herb may be taken before the user is in danger of taking a toxic dose. This makes these herbs relatively safe to use, because there is little chance of overdosing accidentally. However, when an herb has a very low therapeutic margin, it must be used with extreme caution, because it does not take much to go from enough to too much. The therapeutic margin could also be considered, in practical terms, the “margin of error.”

A beginning herbalist, especially, should use with extreme caution – or completely avoid – those herbs which are toxic at relatively small doses and/or accumulate in the body.

David Hoffman¹ offers foxglove (Digitalis purpurea) and the opium poppy (Papaver somniferum) as examples of herbs with extremely low therapeutic margin. These are so potent they are not used in herbal medicine.  (They’re used to produce pharmaceutical drugs, instead, where they can be standardized more precisely.)

Tansy (Tanacetum vulgare) and poke (Phytolacca spp.) are examples of herbs with somewhat higher therapeutic margins that are occasionally used in herbal medicine, but which must be used with extreme caution and are contraindicated in sensitive populations.
¹Hoffmann, D. (2003). Medical herbalism: The science and practice of herbal medicine. Rochester, VT: Healing Arts Press.