General Consideration of Incompatible Prescriptions

Generally, it can be said that the question of compounding and dispensing an incompatible prescription, is entirely dependent upon good judgment.

Prescription of Explosive Substances

• Every pharmacist should know an explosive prescription at sight, should know those which should not be combined under any circumstances (such as glycerin and nitric acid), and those which can be mixed if proper precautions are taken (like iodine and oil of turpentine). Explosive prescriptions are a menace to the Compounder rather than to the customer, hence, self-preservation requires complete knowledge of such dangerous prescriptions.
• In other forms of incompatibility, the pharmacist is confronted with two important questions:

1. Is the prescription safe to dispense?
2. How can it be least objectionably compounded?

Prescription of Internal Incompatibilities Medicines

• There are frequent cases of intentional incompatibilities cases where the doctor really desires an unsightly mixture, because of its therapeutic value. We all know that the official chalk mixture is of value because of the insoluble calcium carbonate it contains, another is the prescription calling for zinc sulfate and lead acetate, which must be dispensed with precipitated lead sulfate since the therapeutic value or its healing property is dependent largely upon it.

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Prescription of Potent Medicines

• On the other hand, prescriptions in which potent medicines are produced in an insoluble form, like prescriptions where powerful alkaloids are precipitated, should be viewed with suspicion by the compounder, and while it is often an exceedingly delicate matter to approach the physician on the subject, it is usually best to let the doctor know, either at the time or later of the possibility of danger. 
• No pharmacist of an act or a good sense of course will bluntly announce to the customer that he will not compound such a prescription, but will always consult the physician. In all cases where precipitation or possible precipitation of potent drugs will take place the container should be provided with a “shake well” label.
• There are some prescriptions of this kind, however, which must not be dispensed, such as the mixing of chloral hydrate and alkali, which will result to the evolution of much chloroform or cases where dangerous or poisonous substances are produced.
• In case the physician is to be approached about dangerous incompatibilities or doses, always approach him armed with accepted authorities like dispensatories and pharmacopeias.
• In a general way it might be stated that when the two substances likely to react are diluted as much as possible before mixing, the chances of incompatibility are minimized.
• Another important question in compounding is “When is the pharmacist justified in Changing a prescription? The answer is that a change is justified only when such change improves the prescription without altering the therapeutic value or action of the preparation, as in the following cases:

1. Aromatic water, when used to dissolve a salt-like potassium bromide, has some of its oil separate, and a cloudy mixture results. If the salt is dissolved in the smallest quantity of water and then mixed with the aromatic water no oil will separate.
2. A prescription calling for phenol mixed with an insufficient amount of water to dissolve it will form a separate layer. But if a part of the water – say 1/8 or ¼ is replaced by glycerin, a perfect solution is formed.
3. A prescription calling for a certain alcoholic tincture mixed with gummy substances may precipitate the gum. If the equivalent amount of fluid extract is used, precipitation may be avoided.
4. Pills calling for ferrous sulfate, USP, or other crystalline salts are found to yield a mass that is too soft. By using the proportionate quantity of exsiccated salt the mass proper consistency can be easily made.
5. A prescription calling for an alkaloidal salt to be dissolved in liquid petrolatum will result in insolubility. But by using the proportionate amount of the alkaloid a good solution will be produced.

As a general rule, it is always best to let the physician whenever a change is made in the prescription, and always explain why the change was made.

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