errors being made in the administration of drugs. Polypharmacy: The use of multiple drugs by a single patient, to treat one or more ailments. 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 effects 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 Epidemiology As of 2008, among US adults older than 56, 4% were taking medication and or supplements that put them at risk of a major drug interaction. Potential drug-drug interactions have increased over time and are more common in the less-educated elderly even after controlling for age, sex, place of residence, and comorbidity. See also Deprescribing Cytochrome P450 Classification of Pharmaco-Therapeutic Referrals Drug interactions can be checked for free online with interaction checkers (note that not all drug interaction checkers provide the same results, and only a drug information expert, such as a pharmacist, should interpret results or provide advice on managing drug interactions) Multi-Drug Interaction Checker by Medscape Drug Interactions Checker by Drugs.com Notes References Bibliography MA Cos. Interacciones de fármacos y sus implicancias clínicas. In: Farmacología Humana. Chap. 10, pp. 165–176. (J. Flórez y col. Eds). Masson SA, Barcelona. 1997. External links Drug Interactions: What You Should Know. U.S. Food and Drug Administration, Center for Drug Evaluation and Research, September 2013 COVID 19 Drug interaction check tool University of