This section is from the book "A Practitioner's Handbook Of Materia Medica And Therapeutics", by Thos. S. Blair. Also available from Amazon: A Practitioner's handbook of Materia Medica and Therapeutics.
Arnica has in the past been employed as a stimulating embrocation to contusions. It has been displaced by more effective applications. When used it should be quite dilute, since it induces erysipelatous inflammations if used in a concentrated state. The 1905 U. S. P. tincture, 200/0, is directed in doses averaging 15 I. internally, but for no very well-defined indications. Arnica is a badly used and consequently much-abused drug. It is an acro-narcotic poison in large doses, and of what legitimate use the usually prescribed doses are in therapeutics is a mystery to most physicians. In small doses it is readily understood so far as theory is concerned, but practically there is a wide variation of opinion as to its uses. Physiologically, small doses act somewhat like phosphorus, and are stimulant to the spinal nervous system. The nerves of the sphincters and of end organs are stimulated, and there is an action not well understood upon the striated muscular fibres. It would thus appear to be indicated in asthenic continued fevers as a nervine. Its known value in paralysis of the optic nerve would suggest its use in other forms of paralysis of peripheral nature and in conditions of threatened paralysis. Its action upon the sphincters seems to be borne out therapeutically in the many favorable reports of its power, in small and frequent doses, of restoring the ability to control the urine and feces. Its action upon the striated muscles has suggested its use in myalgia, pleurodynia, and the muscular pains of overwork and exhaustion, and immediately following childbirth.
Give in doses of I or 2 I. of the tincture.