From pound down to the scruple, the units of the traditional English apothecaries system were a subset of the units of the Roman weight system, although the troy pound and its subdivisions were slightly heavier than the Roman pound and its subdivisions. Similar systems were used all over Europe, but with considerable local variation described below under Variants. The traditional English apothecaries system of weights is shown in the first table in this section: the pound, ounce and grain were identical to the troy pound, ounce and grain (also used to measure weights of precious metals; distinct from the avoirdupois pounds and ounces that were used for measurements of other goods.) The confusing variety of definitions and conversions for pounds and ounces is covered elsewhere in a table of pound definitions. 
To unify all weight systems used by apothecaries, the Irish pharmacopœia of 1850 introduced a new variant of the apothecaries system which subdivided a new apothecaries pound of 12 avoirdupois ounces instead of the troy pound. To allow effective use of the new system, new weight pieces were produced. Since an avoirdupois ounce corresponds to 28.35 g, the proposed system was very similar to that in use in Portugal and Spain, and in some locations in Italy. But it would have doubled the value of the avoirdupois drachm (an existing unit, but by then only used for weighing silk). Therefore, it conflicted with other non-standard variations that were based on that nearly obsolete unit.
The Irish proposal was not widely adopted, but British legislation, in the form of the Medicinals Act 1858, was more radical: it prescribed the use of the avoirdupois system for the United Kingdom (then including Ireland), with none of the traditional subdivisions. This innovation was first used in the United British pharmacopœia of 1864, which recommended the adoption in pharmacy of the imperial avoirdupois pound of 7000 grains and ounce of 437.5 grains, and discontinuation of the drachm and scruple. In practice, however, the old apothecaries system based on divisions of the troy pound, or of the troy ounce of 480 grains, was still widely used until it was abolished by the Weights and Measures Act of 1976, since when it may only be used to measure precious metals and stones. (The troy pound had already been declared illegal for most other uses by the Weights and Measures Act 1878, but this act allowed the sale of drugs by apothecaries weight as an exception.)
Apothecaries' units of any kind became obsolete in the UK with the mandatory use of metric drug measurements from January 1, 1971.
In the United States, the apothecaries' system remained official until it was abolished in 1971 in favour of the metric system.