Underlying Factors
It is possible to take advantage of positive drug interactions. However, the negative interactions are usually of more interest because of their pathological significance and also because they are often unexpected and may even go undiagnosed. By studying the conditions that favour the appearance of interactions it should be possible to prevent them or at least diagnose them in time. The factors or conditions that predispose or favour the appearance of interactions include:
- Old age: factors relating to how human physiology changes with age may affect the interaction of drugs. For example, liver metabolism, kidney function, nerve transmission or the functioning of bone marrow all decrease with age. In addition, in old age there is a sensory decrease that increases the chances of errors being made in the administration of drugs.
- Polypharmacy: The more drugs a patient takes the more likely it will be that some of them will interact.
- Genetic factors: Genes synthesize enzymes that metabolize drugs. Some races have genotypic variations that could decrease or increase the activity of these enzymes. The consequence of this would, on occasions, be a greater predisposition towards drug interactions and therefore a greater predisposition for adverse affects to occur. This is seen in genotype variations in the isozymes of cytochrome P450.
- Hepatic or renal diseases: The blood concentrations of drugs that are metabolized in the liver and / or eliminated by the kidneys may be altered if either of these organs is not functioning correctly. If this is the case an increase in blood concentration is normally seen.
- Serious diseases that could worsen if the dose of the medicine is reduced.
- Drug dependent factors:
- Narrow therapeutic index : Where the difference between the effective dose and the toxic dose is small. The drug digoxin is an example of this type of drug.
- Steep dose-response curve: Small changes in the dosage of a drug produce large changes in the drug's concentration in the patient’s blood plasma.
- Saturable hepatic metabolism: In addition to dose effects the capacity to metabolize the drug is greatly decreased
Read more about this topic: Drug Interactions
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