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.
Arnica flowers are official in all but the Hungarian standards, the leaves in the Croatian and Mexican standards, and the root in the Austrian, Croatian, Italian, Japanese, Serbian, and Spanish standards; yet it is a drug imperfectly understood. In large doses it is a toxic gastro-intestinal irritant which causes a fall in temperature, motor and sensory paralysis, collapse and death. In moderate doses it slows the pulse, slightly raises blood-pressure, and stimulates the vagus nerves (Wilcox).
Externally arnica was formerly much in use, but in many persons its application gives rise to cutaneous inflammation of an erysipelatous type and even to constitutional symptoms. It is now little employed externally.
What rational indication there may be for the "average dose" of 15 grains of the U. S. P. VIII is a mystery I have not been able to solve. The Ninth Revision gives the average dose of the tincture (20%) as 15 minims, which is more reasonable.
In small doses arnica has been recommended in a host of conditions, especially those requiring stimulation of the brain and spinal nerves. It must be admitted that the drug does stimulate the vagus nerve. Now what is the connection here, and what is the pharmacology of the vagus?
The vagus nerve may be involved as the result of neoplasms invading the floor of the skull, or by syphilis, or in palsies from diphtheria toxins or alcoholic excesses. Stimulating the vagus will not help in any of these. Muscarine directly stimulates the vagus nerve endings, and this inhibits the action of the heart. Pilocarpine also does much the same thing, affecting the involuntary muscles and the autonomic and sympathetic nerve-endings; and it also stimulates the plain muscle of certain organs. Atropine, on the other hand, especially in very small doses, annuls the inhibitory effect of the vagus on the heart and the motor action, especially the involuntary. The action of anesthetics on the heart is indirectly through the vagus.
Now then, if arnica stimulates the vagus, the analogies drawn above would lead one to expect it to inhibit excessive muscular action of the heart, to influence the involuntary muscles of the sphincters more particularly (as they are terminal in control), to stimulate the muscular coats of hollow organs, and in a lesser degree to influence other muscular structures. There would be very slight influence upon the nervous system at large except in full dosage.
Therefore it would seem that we can rationally look for an influence from arnica in traumatic strain and in the overworked heart muscle, in atony of the involuntary muscles, in sphincter incontinence, in slight peripheral paralyses, and in muscular exhaustion.
How far this would be clinically borne out no one man can say; but it is unfair to assert that the empiric uses of arnica as a nerve stimulant, in sphincter troubles, in myalgia and muscular strain, in optic paralysis, and in vital exhaustion is wholly unjustified. But I won't attempt to give a definite pharmacology and therapeutics. Any agent of so pronounced a toxicology, so generally used, and so largely recognized in official standards, may not be dismissed lightly.