|METRICATION: 99% complete|
|Patients’ body weights recorded in kilograms||Many patients still weigh themselves in stones|
|Patients’ heights recorded in centimetres||Many patients still measure their height in feet and inches|
|All drug doses in metric|
|All radiotherapy doses in metric|
Prior to metrication, medical practitioners, and dispensers of medicines, had to deal with a confusing array of measurement units using multiple systems of weights and measures. Medicines generally used the Apothecaries’ system of weights and measures, which was based on Troy weights. The Troy pound was different from the imperial, or avoisdupois pound, and was subdivided into a different number of ounces.
It goes without saying that errors in prescribing doses of drugs can have serious consequences, and whenever apothecaries used pounds and ounces, there was always scope for them to be confused with imperial pounds and ounces. If prescriptions were made in imperial units, the calculations needed to convert them to Troy-weights were not straight forward either. In the book Pharmacopoeia Londinensis, published in 1796, the author expresses his frustration with the concurrent use of two measurement systems, “It is to be lamented that the Avoirdupois weights are not banished entirely from the shops of apothecaries”.
Internationally, there was no standardisation of apothecaries’ weights. As recently as the 19th century, across Europe there were more than a dozen different definitions of the apothecaries’ pound.
Apothecaries’ units – mass or weight
|1 ℔||1⁄12 ℔||1⁄96 ℔||1⁄288 ℔||1⁄5760 ℔|
|12 ℥||1 ℥||1⁄8 ℥||1⁄24 ℥||1⁄480 ℥|
|96 ʒ||8 ʒ||1 ʒ||1⁄3 ʒ||1⁄60 ʒ|
|288 ℈||24 ℈||3 ℈||1 ℈||1⁄20 ℈|
|5760 gr||480 gr||60 gr||20 gr||1 gr|
|373 g||31.1 g||3.89 g||1.296 g||64.8 mg|
Apothecaries’ units – liquids
|pint||fluid ounce||fluid drachm||fluid scruple||minim|
|1 pt||1⁄16 pt||1⁄128 pt||1⁄384 pt||1⁄7680 pt|
|16 ƒ℥||1 ƒ℥||1⁄8 ƒ℥||1⁄24 ƒ℥||1⁄480 ƒ℥|
|128 ƒʒ||8 ƒʒ||1 ƒʒ||1⁄3 ƒʒ||1⁄60 ƒʒ|
|384 ƒ℈||24 ƒ℈||3 ƒ℈||1 ƒ℈||1⁄20 ƒ℈|
|7680 m︎||480 m||60 m||20 m||1 m|
|473 mL||29.6 mL||3.70 mL||1.23 mL||0.062 mL|
In the apothecaries’ system, a pound (symbol ℔) was divided into 12 ounces (symbol ℥), and ounces were further subdivided into drams or drachms (symbol ʒ), and scruples (symbol ℈).
A volume of liquid that was approximately that of an apothecaries’ ounce of water was called a fluid ounce (symbol ƒ℥), which was further subdivided into fluid drachms (symbol ƒʒ) and fluid scruples (symbol ƒ℈).
Using the system, numeric values were expressed in Roman numerals. e.g.
℈ix = 9 scruples
ʒiij = 3 drachms (any trailing “i” would be written as “j”)
ʒss = 0.5 drachms (0.5 is represented as “ss”, a Latin abbreviation for “semis”)
ƒ℥iv = 4 fluid ounces
ƒ℈x = 10 fluid scruples
The use of Roman numerals avoided the number ‘3’ being confused with the symbol for drachm ‘ʒ’.
On 3 March 1969, the metric system finally became obligatory for the dispensing of all prescriptions. Bottled medicines were accompanied by 5 mL plastic spoons.
On 1 January 1971, the apothecaries’ system of weights and measures ceased to be legal for trade.
In 1975, the National Health Service adopted SI units for all medical and allied practice.
TOP TIPS :
- Personal height and weight – Measure your height in centimetres, and weight in kilograms. These are the units used by the NHS, and for all other official purposes.
- Baby weights – New born babies are weighed in metric. Weights given in pounds and ounces are conversions from the metric weight. Always ask for the original metric weight of your child, and quote this when asked by friends and relatives. If everyone does this, the habitual use of obsolete pounds and ounces will eventually die out.