This article throws light upon the three main types of incompatibility of drugs in prescription. The types are: 1. Therapeutic (Pharmacological) Incompatibility 2. Chemical Incompatibility 3. Physical Incompatibility.
Type # 1. Therapeutic (Pharmacological) Incompatibility:
It occurs as a result of antagonistic (opposite) pharmacological effects in the patients or interfere with drug absorption following administration e.g., simultaneous administration of physostigmine and atropine in the eye. Physostigmine constricts the pupil, whereas, atropine dilates it. Incompatibility can also occur following oral administration.
Antacids or dairy products will impair the absorption of tetracycline, whereas, kaolin will also produce similar effect when administered with erythromycin. The following are some of the examples of such incompatibility.
CNS: Barbiturates and Strychnine
Heart: Quinidine and Adrenaline
Blood vessels: Adrenaline and Amyl Nitrite
Bronchioles: Histamine and Adrenaline,
Eye: Physostigmine and Atropine
Type # 2. Chemical Incompatibility:
This incompatibility involves some chemical interaction between the ingredients. There may be a chemical reaction, pH changes in dosage forms, interactions with preservatives or solvents in the dosage form, and combination of divalent cations with components of the mixture.
Chemical incompatibility may be manifested in number of ways: precipitation, change in colour, evolution of gas, gelatinization, inactivation without any visible change. It is an extremely bad practice to mix different drugs in the same syringe.
Alkaloids, iodine, arsenic, iron, mercury and silver salts, salicylates, per-magnate, strong acids or alkalis and tannic acid are frequently incompatible with other agents and therefore should not be mixed, knowledge of incompatibility is useful as in the alkaloid poisoning, stomach lavage can be carried out with tannic acid or potassium per-magnate solutions or copper sulphate solution is administered in ingestion of phosphorus.
In some cases chemically incompatible combinations are intentionally prescribed i.e. intentional incompatibility e.g. zinc sulphate and potassium sulphide are prescribed in Alba lotion (white lotion) to precipitate zinc sulphide which is the active agent used in the treatment of acne vulgaris. Chemical incompatibilities may involve different types of chemical interactions e.g. oxidation, reduction, hydrolysis etc.
Type # 3. Physical Incompatibility:
Some physical properties of ingredients interfere in their coexistence. Pharmaceutical incompatibilities are usually of this type.
The following physical properties of ingredients lead to this incompatibility:
When one or more ingredients are not soluble or poorly soluble in the vehicle e.g. Kaolin, calcium carbonate in aqueous solution.
When two liquid ingredients in a preparation are not miscible e.g. Castor oil, liquid paraffin in aqueous solution.
When precipitation of one of the ingredients occurs e.g. volatile oils in aqueous solution if mixed with concentrated salt solution.
(d) Molecular Change:
When two solids are mixed together they may liquify e.g. Acetanilid, acetylsalicylic acid, phenacetin if mixed with resorcinol or urethane. Knowledge of incompatibility is necessary for the veterinarians to avoid unnecessary problems with therapy and to be able to antagonize certain drug effects.
Problems arising due to prescription incompatibility can be avoided by understanding the actions of drugs, taking the help of pharmacist as and when required, refraining from mixing different medicaments in the same syringe and being mean in use of drugs.