This section is from the book "A Text-Book Of Materia Medica And Pharmacy For Medical Students", by Velyien Ewart Henderson. Also available from Amazon: A Text-Book of Materia Medica and Pharmacy for Medical Students.
Extract from the Report of the Council of the British Medical Association on the Adoption of the Metric System of Weights and Measures by Medical Practitioners in Dispensing and Prescribing.
British Medical Journal Suppl.. 1911, 1 p. 205
A. Transitional Procedure Suggested for Adoption by Medical
(5) To practitioners who have been trained according to the present system, the Council recommends the adoption of transitional procedure, which would enable them at once to adapt their prescriptions to the measures of the Metric System, and so avoid the drawbacks that would arise from a divergence in practice between junior and senior practitioners, and would also at once secure for senior practitioners the advantages which make the general adoption of the Metric System desirable.
(6) The difficulty before the practitioner who has been trained to think in terms of grains and minims is to translate his quantities readily into grams and cubic centimetres, and if absolute exactitude were necessary he would require the constant use of tables of equivalents. In practice, however, the most common mode of administering medicines is by spoonfulls, and even when these are poured carefully into a medicine glass the range of variation is relatively wide and the dosage must be such as to make this variation entirely safe. The Council, therefore, feels justified in recommending to the profession, as a transitional measure the following methods which are based on the actual conditions of British practice, and for the suggestion of which the Council is indebted to Dr. R. C. Buist. These will be found to give automatically the conversion of a dosage in grains and minims into a prescription which the dispenser can measure in grams and cubic centimetres with an approximate exactitude well within the range of variation of spoon measures.
(7) In the prescription of an 8 oz. mixture, of which each tablespoonful is to contain
(a) Tr. Belladonnæ, m V. Spt. Ætheris, m X. Vin. Ipecac, m XV. Syr. Scillæ, m XX.
Inf. Senegæ ad 1/2 oz. (i.e., m CCXL). The Metric prescription for the mixture would be
(b) Tr. Belladonnæ, 5 Spt. , Ætheris 10 Vin. Ipecac, 15. Syr. Scillæ, 20. Inf. Senegas ad 240.
On comparing (a) and (b) it is evident that the numbers are the same in both.
(8) The prescriber intends a mixture to contain certain substances in fixed portions, which will be the same in the single dose and in the bulk, and will not be affected, whether the measures be stated in minims or in cubic centimetres; the numbers of minims will be larger, but the proportions will be the same. The exact factors for the conversion of grams into grains and of cubic centimetres into minims are 15,4324 and 16,906, respectively. The procedure used in the above example is to take 16 as a near approximation to each of these numbers. (The extent to which this is inexact may be stated as 4 drops in a teaspoonful.) Now, in ordinary prescribing, 16 doses is the most common of all orders, as represented by tablespoonful doses of an 8-oz. mixture. If, therefore, in such a mixture the prescriber orders the numbers of minims of the drugs A, B, C, D, E in each table-spoonful which he would order in a prescription in English measures, but omits the symbols, and if the dispenser measures in each case the same numbers of cubic centimetres into the bottle, the conversion from English into Metric measures will be automatically completed. Thus it is recommended that the practitioner who wishes to write a prescription for Metric measures should simply write without symbols the drugs with the number of grains or minims he intends to give in each spoonful, and that the dispenser be instructed that each prescription where no symbols are written are to be dispensed in Metric measures.
For teaspoonful doses the bulk would be 2 ozs. or 60 c.c. and for dessertspoonful doses, 4 ozs. or 120 c.c.
(9) The following prescriptions are given in illustration:
(a) Recipe -
Tr. Nucis Vom., 5. Inf. Quass. conc, ad 60. Sig. Teaspoonful in water before each meal.
(b) Recipe -
Tr. Digitalis, 7.5.
Spt. Ætheris, 10.
Dec. Scoparii ad 120.
Sig. Dessertspoonful morn. and night.
(c) Recipe -
Ac. Hydrocyan, dil., 3.
Liq. Morph. Mur., 10.
Syr. Tolut., 30.
Inf. Rosæ Acid. ad 240.
Sig. Tablespoonful thrice daily.
(10) In ordering solutions for various purposes the proportions are so evident that no difficulty arises, and the only point to be borne in mind is the total quantity desired. Thus -
(a) Cocain Hydrochlor., 3 Aq. ad 60. Sig 5% Cocain Hydrochlor.
(6)°Argent. Nitrat., 1. Aq. destil, 50. Sig. 2% Silver Nitrate.
(11) The procedure in ordering pills and powders must be somewhat different from that hitherto described. The order for a pill or powder is based on fractions or small multiples of the grain. The prescriber should therefore become familiar with the equivalence 1 grain =0.06 gram, which is sufficiently exact for practical purposes. To facilitate the work of the dispenser the number of pills or of powders ordered should be a multiple of ten. Thus -
Ext. Hyoscyami aa 0.015. M. ft. pil. M. 10. Sig. One after each meal
For his pill mass the dispenser simply shifts the, decimal point of the prescription.
(12) The equivalence 1 inch =2.5 cm is used in practice.
(13) The procedure here recommended for the use of medical practitioners may thus be summarized:
(a) The prescription is still to be based on the single dose
(b) In the case of mixtures 16 doses are to be ordered by writing with figures only the number of grains or minims of each ingredient in one spoonful.
(c) In the case of pills and powders 10 are to be ordered and the prescription is to give in figures only the metric equivalent of the grains of each ingredient in the single dose.
(d) The dispenser is to be informed that every prescription written without symbols is to be dispensed in Metric measures.
(14) The adoption of the foregoing suggestions would overcome the difficulty of introduction of the new system by a medical practitioner who does his own dispensing, or by one whose dispensing is usually done by the same chemist. For such cases no intervention by the Divisions will be necessary beyond that of bringing this Report under the notice of the local profession. It can be left to each practitioner to take his own course.