Traditionally, the medical profession, chemists and veterinarians used two systems of weights and measures: the imperial or avoirdupois system and the apothecaries’ system.

Systems of measurement are essential. Day-to-day commerce and advancement in areas like science would not exist if not regulated and defined by standards of weights and measures. Systems of measurement presently in use include: the International System of Units (SI), the modern form of the metric system, the imperial system, the apothecaries system and United States customary units.

The apothecaries' system or apothecaries' weights and measures is a historical system of mass and volume units used by veterinarians, physicians and apothecaries for medical recipes, and sometimes during scientific investigation. The apothecary system, originated as the system of weights and measures for dispensing and prescribing medications. The English version divided a pound into 12 ounces, an ounce into eight drams/drachms, and a dram into three scruples or 60 grains. This exact form of the system originated in the United Kingdom and continued in some British colonies into the 20th century.

Until recently, medical recipes were written in Latin, often using special symbols to denote weights and measures.

Weight and volume measurements are especially important to describe the amounts of ingredients used in pharmaceutical recipes (Rx). The apothecary system uses, grains, scruple, drams and ounces in prescribing medications. Pints, quarts and gallons are used to describe larger volumes.

Wholesale druggists and those involved in retail sales involving larger amounts of product (more than an ounce) used the time-honoured imperial weight system and its standard, the 16-ounce pound. Many countries adopted the imperial system, as defined in the British Weights and Measures Act of 1824. The US did so with some modification, primarily to volume units where a US gallon equalled 128 ounces (168 ounce cups) versus 160 ounces imperial (20-8 ounce cups).

For most countries prior to metrification, the imperial system became the standard system of weights used for all goods except precious metals (bullion, troy ounces), precious stones (carats) and medicines (apothecary system). The avoirdupois (imperial) system differs mainly to the apothecary system in that the avoirdupois system exclusively measures weight based on a 16-ounces pound versus 12-ounces in the apothecary system. Smaller quantities of pharmaceutical ingredients, such as those used in formulating prescriptions, were prepared using an apothecaries’ system of measurement (grains, scruples, drams).

Apothecaries’ measure of weight: 1 grain (gr.)

1 scruple (℈) 20 grains

1 dram (ʒ) 3 scruples 60 grs (Eng.drachm)

1 ounce (℥) 8 drams 480 grs

1 pound (lb.) 12 ounces 5760 grs

The metric system (milligrams, grams, kilograms, liters) now exists in most countries. The US customary system (USCS or USC) emerged from English imperial units, used in the British Empire before the US gained independence. The US overhauled its system, modelling it after the imperial system, yet changing the definitions of some units like volume. France introduced the metric system in 1799. The metric system is a decimal system, making it easier to learn than the non-decimal US system.

Today, pharmacists use combinations of several systems to write, prepare and dispense medications.

Between the decline of the Roman Empire and metrication, measure and weight systems varied across Europe depending on the purpose being served. International commerce necessitated the need to adopt and understand weighing and measuring practices used by trading partners.

In the 19th century, most European countries and cities had at least a "commercial" or "civil" system (such as the English avoirdupois system) for general trading, and another system (such as the troy system) for precious metals like gold and silver. The "carat" (1 carat = 20 mg) remained the standard for weighing gemstones and pearls.

The apothecaries' weights and normal commercial weights could vary depending on the systems in use.

The differences caused confusion. Conflicts were not uncommon in business between merchants and apothecaries. If the system used to weigh drugs as the apothecary prepared prescriptions differed from weights used when prescriptions were dispensed, confusion arose as did doubt about practices of unscrupulous apothecary owners. Fortunately, the trust communities developed with doctors, veterinarians and their druggists became golden and constituted a priceless quality that persisted in frontier society.