This section is from the book "Botanic Drugs Their Materia Medica, Pharmacology and Therapeutics", by Thomas S. Blair. Also available from Amazon: Botanic Drugs, Their Materia Medica, Pharmacology and Therapeutics.
Wintergreen, Gaultheria procumbens. The Oil of Gaultheria was official in the Eighth U. S. P. Feebly antiseptic. Average dose 15 minims in capsules. Used in the treatment of rheumatism; but, more especially, as an external application.
Oleum Betulae, Sweet Birch, Betula lenta, is used similarly.
Methylis Salicylas, Methyl Salicylate, a synthetic ester, is chemically similar and is largely used externally. It is much cheaper than the true oils and serves well for external uses; but it is open to question if it serves so well for internal administration. I do not believe it does.
But the internal use of none of these agents is to be commended; they soon become insupportable on account of their high flavor. The salicylates are certainly to be preferred. For a discussion of the therapeutics of these agents see "Salicylic Acid."