This section is from the book "Practical Materia Medica And Prescription Writing", by Oscar W. Bethea. Also available from Amazon: Practical Materia Medica and Prescription Writing.

In writing the inscription a matter of some embarrassment to the beginner is calculating the total amount to order of each ingredient.

The usual method is to write first the name of each drug, then decide on the number of doses in the prescription, and by making the vehicle q. s. to the desired bulk, or ordering the desired number of capsules, etc., and writing the directions, get the number of doses fixed before the writer; then beginning with the first drug multiply the amount desired for each dose by the total number of doses of the finished product.

For example:

Elix. Aromatici.......................................................... | q. s. |

Teaspoonful in water every three hours.

Two fluidounces being the total quantity and a teaspoonful the dose, the number of doses would be sixteen. If ten grains of Sodium Bromide are desired at each dose, the amount would be 16 times 10, or 160 grains.

Or:

9 |

Quininae Sulphatis. ................................................................ |

Ferri Reducti. ............................................................... |

M. ft. cap. no. xx.

One after each meal.

If it is desired to give two grains of the quinine salt at a dose and there are twenty doses, the amount of the salt would be 20 times 2 grains, or 40 grains.

In ointments, etc., the amounts are usually based on per cent.;

Phenolis, | |

Petrolati........................................................................ | q. s. |

M.

Apply twice daily.

If it is desired to use about 1 per cent. of Phenol the calculation is 1 per cent. of 480 grains, or 4.8 gr. (or about 5 grains) total quantity of phenol.

A method sometimes used for calculating approximate amounts is as follows:

Base the calculations on an 8-fluidounce prescription with tea-spoonful doses. This would give about 60 doses to the entire quantity. For each ingredient write for as many drachms or fluidrachms as it is desired to give grains or minims at a dose.

For example:

Sodii Iodidi, | |

Elix. Aromatici............................................................ | q. s. |

M.

Teaspoonful in water three times a day.

This would give about 60 doses, and as a drachm is 60 grains, each dose will contain about as many grains as there are drachms in the 'total quantity of the salt. If it is desired to give 5 grains at a dose write for 5 drachms of the salt.

A 4-fluidounce prescription would naturally require one-half the number of drachms or fluidrachms as grains or minims were desired at a dose. Fluid prescriptions of any size may be adjusted on this basis. The same rule would apply in writing for 60 pills or capsules or greater or less amounts adjusted as in the case of fluids.

In metric prescriptions the following excellent scheme has been suggested:

A gramme or a mil contains about 15 or 16 grains or minims respectively. The slight difference is immaterial. In ordering fluid preparations base the calculations on a 60 mil quantity (about 2 fluid-ounces) with teaspoonful doses; this will give about 16 doses. For each item (except, of course, the vehicle) write for as many grammes or mils as it is desired to give grains or minims at a dose. For example:

Sodii Bromidi, | ||

Elix. Aromatici...................................................... | q.s. 60 |

M.

Teaspoonful in water every three hours.

As this would contain about 16 doses and a gramme is about 16 grains, for each gramme of the salt ordered the patient would get a grain at a dose. So if it is desired to give ten grains of the salt at a dose, write for ten grammes in the inscription.

Larger or smaller prescriptions can be arranged on the same basis.

In ordering capsules, pills, etc., if the prescription calls for 15 or 16 the patient will get as many grains in each dose as there are grammes in the total quantity. Prescriptions for greater or less number can, of course, be calculated on the same basis. The scheme is too simple and its advantages too obvious to require discussion.

It is a safe plan to calculate the amounts by one of the methods mentioned and, in rechecking, to use another, as this reduces the chance for error to a minimum.

Continue to: