R.M. Armstrong was a district surgeon in the Cape Colony in 1845, when he requested permission to practice as a chemist and druggist. He was licensed as such in October that year. Ten years later he was a medical practitioner in Grahamstown. On 3 July 1855 he was one of five medical men who founded the Grahamstown Medico-Chirurgical Society (soon renamed the Literary, Scientific and Medical Society). At the third meeting of the society, in August 1855, he read a paper on the medicinal qualities of Cannabis indica (dagga), a plant that grew all over the Cape Colony and was used as a soporific and antispasmodic. He explained the variable medical results obtained with extracts of the plant by his finding that only female plants contain the active substance. In January 1856 he read a paper to the society on the pharmacological uses of animals, dealing mainly with beetles of the family Cantharidae (Spanish flies, or blister beetles). His attention was drawn to local species of the family by a friend living in Cradock, whose crops had been devastated by them and who reported that a crushed beetle had raised a blister on his skin. Armstrong identified the beetles as belonging to the family Cantharidae, extracted the active substance with boiling vinegar and found it to be a powerful agent for vesication (producing blisters). He further noted that the pharmacological effect of his extract was more pronounced than that of extracts obtained from Europe and ascribed such differences to the beetles' diets. His enquiries indicated that the extract could be economically prepared from local beetles to replace the imported product. The substance had several medical uses at the time, for example in the treatment of ulcers and inflamatory diseases.
Armstrong presented a bi-weekly class on botany for the Literary, Scientific and Medical Society during 1856, and in November that year read a paper to its members on medicine versus quackery. He was a member of the first executive committee of the Eastern Province Agricultural Association, which was founded in November 1855. By 1862 he was no longer listed as a medical practitioner, but in 1885 he was still listed as a licensed chemist and druggist in the Cape Colony.