Although alkaloids are naturally occurring bases, additional generalizations are difficult because they include a wide range of structurally dissimilar compounds. They vary greatly in their chemical and physical properties as well as in their distribution in nature. In some cases certain alkaloids are associated with only a single species of plant, whereas others are more widely distributed between biological groupings of plants, and yet others are found in a wide range of unrelated plants. Likewise, the concentration of a particular compound may be highly localized within a given region of one plant and found predominantly in a different region of another plant. In some cases the levels of an alkaloid may be relatively high and its isolation as a natural product may be economically feasible, whereas in other cases the levels may be extremely low and less commercially desirable. The medicinal use of alkaloids in the form of crude plant extracts has been known for several thousand years and today there are hundreds that have been isolated and characterized. However, very few of them are accepted therapeutically and many of these fit into the broad categories of cinchona, ergot, opium, rauwolfia, tropane, vinca and xanthine alkaloids.