To acquire careful habits it is wise, in writing a compound prescription, to put down the names of all the ingredients desired before inserting the quantities. Then multiply the number of doses by the desired dose, and set down the result opposite the name of the ingredient. Total quantities are usually expressed in the nearest half or whole number rather than in fractional amounts, the error in such a case being small in proportion to the whole amount of the dose.

In a liquid prescription the name of the vehicle always comes last, followed by q. 3. ad and the total quantity of the prescription.

A number of ways to promote ease in the calculations have been suggested. A one-ounce mixture may be reckoned as eight teaspoonful doses, a two-ounce as 15 teaspoonfuls, a three-ounce as 24 teaspoonfuls, and a four-ounce as 30 teaspoonfuls.

Hence a two-ounce bottle contains 15 or 16 teaspoonfuls; a four-ounce bottle contains 15 or 16 dessertspoonfuls; an eight-ounce bottle contains 15 or 16 tablespoonfuls.

One simple rule is: For an eight-ounce mixture with tea-spoonful dose prescribe as many drams of the ingredient as you desire minims or grains at a dose; for a four-ounce mixture, half as many drams; for a three-ounce mixture, two-fifths as many, and for a two-ounce mixture, one-fourth as many. In other words, in a two-ounce mixture with teaspoonful dose one dram of the substance gives a 4-grain or minim dose; in a three-ounce mixture one dram gives a 2 1/2-grain or minim dose; in a four-ounce mixture one dram gives a 2-grain or minim dose.

Example: The single dose of the prescription being -

Figuring The Quantities 147 Ammonii chloridi ..................................

........gr. v

Syrupi ipecacuanhae .............................

Figuring The Quantities 148 viij

Aquae ....................................................

q. s. ad 3 j

2-Ounce Mixture

3-Ounce Mixture

4-Ounce Mixture

1 1/4 drams............

gr. lxxv

3 ij

3 iiss

2 drams .........................

...3ij

3 iij

3 iv

to 2 ounces ........................

adFiguring The Quantities 149 ij

adFiguring The Quantities 150 iij

adFiguring The Quantities 151 iv

Observe that increase in size of mixture requires increase in amount of active ingredients. Increase in dose of mixture requires decrease in amount of active ingredients.

Where the ordinary spoons are to be used and not a measuring-glass, a method recommended by De Lorme is to assume six teaspoonfuls to an ounce and follow this rule: "Employ 1/2 dram to each ounce for five-grain or five-minim doses in each teaspoonful." This does not apply to preparations for external use, i. e., those not measured by the spoon.

There is a method advocated by some, of figuring out the doses in the English system, but writing the prescription according to the metric system. The rule is to write always for sixteen doses, i. e., a two-ounce mixture (written 60 c.c.) if the dose is a teaspoonful, a four-ounce mixture (written 120 c.c.) if the dose is a dessertspoonful, an eight-ounce mixture (written 240 c.c.) if the dose is a tablespoonful. Then put down for each ingredient as many grams or cubic centimeters as you desire grains or minims per dose. The above prescription by this method would read -

Figuring The Quantities 152 Ammonii chloridi.......

....... 5.0

Syrupi ipecacuanhae...........

....... 8.0

Aquae......................................................

q.s. ad 60.0

Sixteen powders or pills or capsules may be prescribed in the same way; eight powders would require half as many grams as grains per dose, etc. This is an easy method for older doctors who know their doses in the English system, and desire to make their prescriptions conform with the metric system. But as it requires thinking of doses in grains and minims, and yet writing in metric amounts, it is an unwise method for a student to learn. If he is going to write metric prescriptions, he had better learn his doses at the outset in the metric system.

In prescriptions for children a simple application of the author's age-weight rule for dosage (see Part I) is to make the prescription for two ounces with teaspoonful dose, and to put down for each ingredient half as many grains or minims as its adult dose, multiplied by the age of the patient plus 3. Thus, for a child of two years the prescription above would read:

Figuring The Quantities 153 Ammonii chloridi ..............................................

gr. xij

Syrupi ipecacuanhae ........................................

Figuring The Quantities 154 xx

Aquae ...................................................q. s. ad

Figuring The Quantities 155 ij

If using Cowling's rule, the prescription may be a three-ounce mixture with teaspoonful dose, i.e., 24 doses. Then the adult dose multiplied by the age at next birthday will be the total amount. For a child of two it would read:

Figuring The Quantities 156 Ammonii chloridi....................

gr. xv

Syrupi ipecacuanhae ............................................

Figuring The Quantities 157 xxiv

Aquae ........................................................q. s. ad

Figuring The Quantities 158 iij

For 12 doses it would read half these amounts.

"Lazy Man" Prescriptions. - The method of writing bulk prescriptions, by putting down the single dose of each ingredient and directing the pharmacist to send a certain number of such doses (mitte tales doses), is known as the "lazy man's method," and is not approved. Such a method is good usage only in prescriptions for objects of standard formula, such as pills, capsules, etc., which are understood to be kept ready made by the pharmacist. (See Types of Prescriptions.)

A shot-gun prescription is one that contains a number of substances which have no essential therapeutic affinity. It is the result of an ignorant attempt to hit the trouble, no matter what may be its nature. Warburg's tincture is a good example of such.