Factors Modifying Drug Action
Following factors or conditions pre-dispose or favour the appearance of drug interactions:
• Old age
It is known that liver metabolism, kidney function, nerve transmission or functioning of bone marrow decreases with age. In addition, in old age there is a sensory decrease which increases the chances of errors being made in the administration of drugs.
More the number of drugs a patient takes, more are the chances amongst their interaction.
• Genetic factors
Genes synthesise enzymes which metabolise drugs. Some races have genotypic variations which could decrease or increase the activity of these enzymes. The consequence of this effect predisposes the patients towards more drug-interactions. Genotypic variations in isozymes of cytochrome P450 is a classical example.
• Hepatic/renal diseases
Drugs which are metabolised in liver and/or eliminated by kidneys can have pharmacokinetic alterations if liver and/or kidney is not functioning properly. In such cases serum concentrations of related drugs may increase, because of which the patients may show adverse reactions/interactions.
• Serious diseases
For any reason, if dose of the drug is reduced, there is a likelihood that serious diseases could worsen.
• Drug-dependent factors
Factors like narrow therapeutic index (e.g. digoxin), steep dose response curve, saturable hepatic metabolism as in case of phenytoin, can alter effects of drugs at different concentrations.