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.
Mountain Laurel, Kalmia latifolia. This plant contains arbutin. Kalmia is not official, nor is it a prominent drug; but it belongs to the Ericaceae, the properties of which have been discussed under "Epigaea" and "Chimaphila," q. v. The Ericaceae generally are diuretic, due principally to the content of arbutin. Uva-ursi is of the same group; so is Sourwood, Oxydendron arboreum, a pleasant, acidulous diuretic used largely for the removal of dropsical effusions (fl. 5 to 20 minims). Ledum lati-folium, Labrador Tea, another diuretic, as well as the allied Ledum palustre, Marsh Tea, belong to this group. Kalmia, however, is not an acceptable diuretic, as it contains very little arbutin and has other principles of a disturbing nature.
Gaultheria, q. v., Stagger-bush or Andromeda Mariana, and Kalmia are also Ericaceae. Gaultheria is important on account of its oil, but it also contains arbutin. Stagger-bush causes the death of many lambs and calves that eat it. The allied Kalmia angustifolia and Kalmia latifolia are nearly similar in action and are also reputed to kill small animals. Andromedotoxin is said to be the toxic agent involved.
I doubt the truth of this, at least as regards Kalmia. Sourwood, referred to a paragraph back, is an andromeda, and it is not poisonous; but the Andromeda Japonica, as grown in Japan, contains andrometoxin, which may or may not be the same thing as andromedotoxin. The azaleas and rhododendrons are also said to contain it; and it is alleged to be the toxic principle of the Ericaceae generally. Now just bear in mind that huckleberry, blueberry, cranberry, heather, sand myrtle, beetle-weed, Indian pipe, beech-drops, and a veritable host of other plants are Ericaceae, very few of which are toxic and just an occasional one of medicinal interest. I am inclined to the view that methyl salicylate figures more than does andrometoxin (or andromedotoxin) as an ericaceous toxic agent.
Kalmia latifolia has been recommended as a remedy in syphilis, fevers, hypertrophy of the heart, and a host of other affections. Homeopathic texts recommend it in rheumatism and locomotor ataxia. It is used from the "200th potency" to 5 minims fl. I have tried it out in a number of cases and failed to find it of any value in any of its indications. The so-called "provings" of it failed utterly to materialize when I took a number of full doses myself. I have been much in the mountains and have eaten the leaves of these laurels, rhododendrons, and azaleas repeatedly, and my horse would sometimes sample them also. Nothing ever came of it. But there does seem to be some justification for the common belief that some of these bushes are poisonous to sheep. So are a lot of plants in the far West that seem to be otherwise quite innocuous. Space is given here to the matter because of its general interest.