When the response to one or more drugs in the patient is of a nature or intensity different from that intended, it is known as therapeutic incompatibility. It is within the realm of the physician and mainly concerned with prescribing errors and post-administration effect. Some of the adverse drug reactions may be considered as examples of therapeutic incompatibilities.

Drug interactions have been dealt with in a separate chapter in this book. Common types of therapeutic incompatibilities are due to:

A. Dosage errors:

Dispensing of an overdose of a prescription constitutes the most serious type of dosage errors. Any experienced pharmacist should be able to detect such an error while checking the prescription.

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B. Wrong dose or dosage form:

As many drugs have confusingly similar names, there is always a possibility of dispensing the wrong drug e.g., prednisone and prednisolone, protamine and protamide, digoxin and digitoxin.

Similarly many dugs are available in different dosage forms and hence if the dosage form is not mentioned clearly on the prescription, the pharmacist must seek clarification from the prescriber. It is highly desirable that the pharmacist should be aware of the potential hazard of such errors.

C. Contraindicated drugs:

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Certain drugs may be contraindicated in a particular disease or when a particular patient is allergic to it. Thus corticosteroids are contraindicated in patients having an active peptic ulcer. Morphine, barbiturates, and related drugs may be dangerous in severe asthma. Vasoconstrictors should not be given to any hypertensive patient.

E. Synergistic or antagonistic drugs:

Prescribing of synergistic or antagonistic drugs may be either deliberate or by mistake. The activity of penicillin derivatives may be prolonged by the use of probenecid and hence this combination is advantageous therapeutically as well as economically.

Prescribing an amphetamine with a babiturate is apparently antagonistic combination but has been accepted in modern therapeutic practice to help in weight control.

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Some other antagonistic combinations would result in prescribing stimulants with sedatives, cholinergic drug with anticholinergic drug, purgatives with antidiarrhoeals etc. Concurrent use of an antacid with tetracycline will lead to decreased absorption of the antibiotic.

In conclusion it must be emphasized that although incompatibility continues to pose a serious problem in drug therapy, an experienced pharmacist should be able to detect and reasonably correct the same through his professional knowledge, skill and common sense. Drug interactions also lead to incompatibility.