Part 59. Medicines and poisons. It is an old saying that medicines are substances which make the sick well and the well sick. This saying expresses in a way the truth that among medicines are included some of the most powerful poisons known. In fact, most medicines are poisonous, and most poisons medicinal. Experience has shown also that when a fatal dose of a certain poison has been taken, life may sometimes be saved by giving, as an antidote, some other poison in quantity sufficient even to cause death if the first poison had not already been taken.

From these facts it appears that no line of separation can be drawn between medicines and poisons. By a medicine we mean any substance used for the cure or relief of disease; and by a poison, any substance capable of injuring the body by other than mechanical means so as to cause death or serious harm if taken in undue quantity. Even too much food may be harmful or perhaps fatal, and the same is true of the most harmless medicines, but in these cases the bad effect is so largely the mechanical result of excessive quantity that we do not say poisoning has taken place. Foods are sometimes used as medicines, as, for example, olive-oil and Irish moss. The same is true of food-adjuncts in general, and, as we have already seen, many of these if taken in more than small amount are poisonous. We may recall also the fact that certain foods, such as tapioca, are obtained from plants which contain deadly poisons. Similarly the tubers of the white potato when young or when green in color, contain a powerful poison. Thus it is plain, that edible, medicinal, and poisonous plants must not be thought of as entirely separate and distinct classes, but merely as groups made for practical convenience.

The number of plants which have been used medicinally is enormous. Many of these, however, have been found to be either so dangerous in their action or of so little value that they are now used if at all only by the ignorant. Nevertheless, the number of those still used in scientific medicine is rather large. Numerous also are the poisonous plants known to botanists. Plainly, in the present chapter only a small proportion of these can be considered. The ones chosen are typical examples of those classes of medicinal and poisonous plants about which it is most important for a beginner to know. The medicinal plants are thus divided: (a) those yielding non-poisonous drugs, and (b) those yielding poisonous drugs. Poisonous plants are grouped into (a) those dangerous to eat and (b) those dangerous to handle.