This section is from the book "Materia Medica: Pharmacology: Therapeutics Prescription Writing For Students and Practitioners", by Walter A. Bastedo. Also available from Amazon: Materia Medica: Pharmacology: Therapeutics: Prescription Writing for Students and Practitioners.
Liquid medicines for internal use are administered by measure only, hence it is the custom to make the total quantity of the prescription such that its dose will be a teaspoonful, a dessertspoonful, or a tablespoonful, regardless of the amount of active ingredients present. The difference between the measure of the active ingredients and the measure of the dose is made up by the vehicle. It is for this reason that in this country we measure liquids instead of weighing them, and vary the amount of the vehicle or diluent as needed to make the total the number of readily measured doses desired. Thus of the vehicle we employ "q. s. ad iv," i. e., as much as may be sufficient for four ounces (or whatever total quantity is desired), regardless of the amount of active ingredients present.
The necessity for this may be illustrated by the following prescription. If we wish to give 10 minims of the tincture of nux vomica at each dose in the following bitter appetizer and tonic mixture, we should write:
Tinct. nucis vomicae ..........................................................
Tinct. cardamomi comp ..........................................q. s. ad
M. et Sig
One dram in water t. i. d. a. c.
This calls for 24 doses, containing 240 minims of the tincture of nux vomica, i. e., each dose contains 10 minims. If this should be written:
Tinct. nucis vomicae..................
Tinct. cardamomi comp .........................................
the total quantity of the prescription would be iiiss, or 28 doses, and each dose of the tincture of nux would be 8 4/7 minims. Another reason for avoiding this last type of prescription is that the quantities make an irregular total, and do not fit any standard sized bottle.
The measures used by patients are: drop, teaspoon, dessertspoon, tablespoon, sherry glass, wineglass, tea-cup, and glass or tumbler.
Drops are uncertain measures, their size differing according to the viscosity of the liquid, the temperature, the fulness of the container, the surface from which dropped, the rapidity of dropping, etc. Drop bottles and medicine-droppers or pipets may be had, but these vary greatly in the size of their orifices, and consequently in the size of their drops. For example, with five medicine-droppers bought at different drugstores by the writer, 60 minims of the tincture of nux vomica required respectively 200, 172, 167, 142, and 132 drops, while from the shop bottle containing the tincture it took 125 drops. Of commercial droppers, the only one that we know that is made with a standard orifice is the Barnes Medicine Dropper (not the Barnes Eye Dropper). With this dropper 60 drops of water measure 60 minims; of other liquids the number of drops varies according to their nature. The drop is, therefore, not a certain measure. We have several times prescribed the Barnes Medicine Dropper and found that the druggist sent instead a dropper with a much smaller orifice.
Approximately, when dropped from the mouth of a bottle, aqueous liquids, glycerin, and the fixed oils measure one drop to the minim, volatile oils and strongly alcoholic liquids 2 drops to the minim, ether 3 or 4 drops, chloroform 5 drops, and bromoform 6 drops.
The term minim should not be used in the directions for the patient unless the patient or nurse has a minim glass for accurate measuring.
A medicinal teaspoonful is 1 dram, a dessertspoonful is 2 drams, a tablespoonful is 4 drams; but, unfortunately, the spoons in common use are not made to standard, and hold from 25 to 50 per cent. more than these amounts. Hence if accuracy is important, it is a good plan to advise the use of measuringglasses, which may be had at trifling cost correctly graduated on the scale of one dram to one teaspoonful. In lieu of the measuring-glass, DeLorme suggests that we reckon on six teaspoonfuls to an ounce; and he shows how much such a procedure tends to simplify the calculation of quantities in prescriptions. (See below.)
A sherry glass holds about 2 ounces, a wineglass about 3 ounces, a glass or tumbler about 8 ounces. A tea-cup holds 5 or 6 ounces.