Occasionally, the drugs we use to improve a person's condition may not work in the manner intended. The outcome may be contrary to that which was expected, and, indeed, could even cause harm to the patient. It is important to be aware of symptoms that may indicate a drug is not doing its job properly.

Incompatibilities
LEARNING OBJECTIVE: Identify the three classifications of drug incompatibility, and recall what causes these drug incompatibilities to occur.

There are instances when a drug used simultaneously with another drug or substance does not perform as it was intended. These drugs or substances may be incompatible together and, therefore, should not be administered at the same time. A drug incompatibility can also occur when drugs are compounded together in the pharmacy. There are three classes of drug incompatibilities: therapeutic, physical, and chemical. In the following sections, each class of drug incompatibility is discussed.

THERAPEUTIC INCOMPATIBILITIES.- Therapeutic incompatibilities occur when agents

Figure 6-2.-Pharmaceutical instruments.

antagonistic to one another are prescribed together. Such circumstances seldom occur, but when they do, the Hospital Corpsman should bring the perceived incompatibility to the attention of the physician. The pharmaceutical agents may have been used together for one agent to modify the activity of the other. The physician will verify the prescription as necessary.

PHYSICAL INCOMPATIBILITIES.- Physical incompatibilities are often called pharma- ceutical incompatibilities and are evidenced by the failure of the drugs to combine properly. It is virtually impossible for uniform dosages of medicine to be given from such solutions or mixtures. Ingredients such as oil and water (which are physically repellant to each other) and substances that are insoluble in the prescribed vehicle are primary examples of physical incompatibilities.

CHEMICAL INCOMPATIBILITIES.- Chemical incompatibilities occur when prescribed agents react chemically upon combination to alter the composition of one or more of the ingredients (constituents).

MANIFESTATIONS OF INCOMPATIBI ITY.- The following list outlines the various ways incompatibility between or among drug agents may be manifested. The respective type of incompatibility is also noted.

Insolubility of prescribed agent in vehicle (physical)

Immiscibility of two or more liquids (physical)
Precipitation due to change in menstrum that results in decreased solubility (called salting out) (physical)
Liquification of solids mixed in a dry state (called eutexia) (physical)

Cementation of insoluble ingredients in liquid mixtures (physical)
Evolution in color (chemical)
Reduction or explosive reaction (called oxidation) (chemical)

Precipitation due to chemical reaction (chemical)
Inactivation of sulfa drugs by procaine HCl (therapeutic)
Although it is, of course, impossible to eliminate all drug-agent incompatibilities, some combinations may respond to one of the following corrective measures.

Addition of an ingredient that does no alter the therapeutic value (such as the addition of an ingredient to alter solubility of an agent)
Omission of an agent that has no therapeutic value or that may be dispensed separately

Change of an ingredient (e.g., substitution of a soluble form of an ingredient for an equivalent insoluble form)
Change of a solvent
Utilization of special techniques in com-pounding