i think its not commonly used as it is rarely sold. For instance, the yellow rhododendron (Rhododendron chrysanthum, family N.O. Ericaceae) can only be found in the mountains of Siberia. But Russian folk rememedy swears on its high potency as across the board painkiller, even for the worst type of gout or rheumatism. And there are other kinds as well, such as the more common Rhododendron ponticum (Rhododendron ponticum leaves exhibited potent anti-inflammatory activity even in lab animal tests. This is a popular Turkish medical herb trusted by many because it doenst induce any gastric irritation or damage.
I shall paste hereafter some of my notes on it (hope yall dont mind, its a bit longish, but since you asked...):

Yellow rhododendron (Rhododendron chrysanthum)
Description: A small bush, stem 1 to 1 1/2 foot high, spreading, much branched, often concealed by moss, tips of shoots only being visible. Leaves alternate like laurel, ovate, somewhat acute, tapering to stalk, reticulated, rough above, paler and smoother underneath. Flowers large, showy, nodding, on clustered terminal, loose peduncles emerging from large downy scales. Corolla campanulate, five cleft, rounded segments, three upper largest and streaked with livid dots next the tube, lower unspotted. Stamens ten, unequal deflexed; anthers oblong, incumbent, without appendages, opening by two terminal pores, capsule ovate, rather angular,five-celled, five-valved, septicidal; seeds numerous, minute. The leaves should be gathered directly the capsules have ripened. They have a faint odour when first gathered and a bitter, acrid, astringent taste.

Constituents: The leaves contain a stimulant narcotic principle, which they yield to water or alcohol

Medicinal Action and Other Uses: Aside from pain and swelling control, a tincture of the fresh leaves is said to be curative of diarrhoea, amenorrhoea, chorea, affections of the eyes and ears, and neuralgia. Much used in the former USSR and central asia as a remedy for severe rheumatism. Also useful for syphilis.

Dosage: 2 teaspoonsful of the infusion.

Im no authority on this! Please ask an expert in your vicinity who might be able to provide more and clearer information. And besides, check on the availability of rhododendron in your area. In England where its not endemic, the rhododendron ponticum population has become too big that its now considered a major pest weed.

cheers