I’ve been poking trhough William Heberden’s Commentaries on The History and Cure of Diseases (1802).
Chapter 62, Menstrua, includes the following:
“The pains, which several women experience during some part of the menstrual flux, are safely mitigated with opium; and such persons should always have in readiness half a grain or a grain of opium, to be taken as soon as the pain comes on, and to be repeated once or twice if the pain require, at the distance of half an hour. This has been very frequently given without checking, or in any manner deranging this evacuation.
To those, whose stomach will not bear opium, it has been given as safely in a clyster. The tincture of opium has not appeared to be without some effect, when only rubbed in by a warm hand over the abdomen. Warm bathing, sitting over the steam of warm water a few mornings before the expected return of the catamenia, Bath water both externally and internally, have all been employed against this complaint, and with advantage.”
Googling tells me that a grain of opium (0.05 scruples) is equivalent to 25 drops of laudanum, half an ounce or so of paregoric or Godfrey’s Cordial, or ten grains of Dover’s Powder.