Drug Interactions: A Comprehensive Update

Possibility of occurrence of drug interactions increases with the number of drugs administered. Drug
interactions can be classified in a number of ways using different basis as well as terminology. Drugs
may interact with other drugs, foods, beverages, contrast media, and herbs; outside or inside the
body. The term “drug-condition interaction” is sometimes used when the existing medical condition
makes certain drugs potentially harmful. Knowledge of In vitro interactions is essential to avoid loss of
activity of drugs before administration. Although every theoretical drug interaction may not manifest in
practice, “drug interactions” is a prominent cause of adverse or undesired events related to drug
administration or treatment failure. Amongst the herbs, St. John’s wort has a potential of producing
significant drug interactions due to its capacity to induce metabolism of number of drugs. In vivo
interactions at pharmacokinetic level affect absorption, distribution, biotransformation or excretion of
drugs. Induction or inhibition of cytochrome P450 (CYP450) enzymes forms a major basis of drug
interactions. As compared to induction, inhibition is a fairly rapid process. Number of precipitant drugs
which inhibit the metabolism is much more than those that produce induction of enzymes; hence
inhibition of metabolism may lead to serious and acute adverse events by aggravating the toxicity of
substrate drugs. Role of drug transporters, especially P-glycoprotein (P-gp), in causation of drug
interactions is being increasingly identified. P-gp affects absorption, distribution and excretion, and
hence plays a major role in pharmacokinetic drug interactions. Additionally, P-gp works hand in hand
with CYP450 enzymes. In pharmacodynamic interactions, the drugs synergise or antagonise the
effects at the level of target of action. Clinically beneficial and reparative drug interactions are
explored to obtain useful drug combinations. Interactions of drugs with contrast media should be
remembered and carefully prevented. Extensive research has led to the development of a large
number of In vitro and In vivo methods to detect and predict drug interactions. “Drug interaction
softwares” is an additional and significant tool for analysis and prediction of probable drug
interactions. Appropriate awareness and knowledge of possible drug interactions is crucial in
prevention of harmful drug interactions and their consequences.