Even though researches and studies are being conducted on ‘drug-interactions’, similar investigations are required to be done on other types of interactions such as drug-herb interaction, drug-food interaction, drug-device interaction etc. to determine the efficacy of the drug.

The researchers on drug interactions should keep in mind that patients may take non-prescription substances or get exposed to foodstuffs or therapeutic devices which may alter the effects of drugs that they have prescribed or dispensed, according to Dr J K Aronson, president Emeritus, British Pharmacological Society, Oxford, UK.

An interaction occurs when the effects of one drug are altered (increased or decreased) by the effects of another drug (the precipitant or perpetrator drug). Among 162,744 reports of suspected adverse reactions reported to the US Adverse Event Reporting System (AERS) in 2008 there were 1167 multi-item associations, only 4 per cent of them were drug–drug interactions, he said in a message sent to Dr Dixon Thomas, Head of Dept of Pharmacy Practice, Raghavendra Institute of Pharmaceutical Education and Research (RIPER) in Andhra Pradesh.

According to Dr Aronson, a study conducted on 75 patients in an oncology unit in south India, there found 213 interactions, of which 21 were major, 121 were moderate and 71 minor. The reports on clinically significant interactions between anticancer drugs were 13 (6.1 per cent) and 14 (6.5 per cent) cases were reported on interactions between anticancer drugs and other drugs prescribed for co-morbidities. A herb, a food, or a device can also play the part of the precipitant or perpetrator in a drug interaction, but the incidence of such interactions is not known, he said in the letter sent to the research institute.

In drug–herb interactions the herbal preparation is the precipitant. Although herbal medicines are often regarded by the people as not only effective but also perfectly safe, many examples of adverse reactions and interactions have been reported. The best known example of a herbal precipitant is Saint John's wort (plant species ‘Hypericum perforatum’) which can induce the metabolism of amitriptyline, cyclosporine, digoxin, HIV protease inhibitors, irinotecan, oral contraceptives, phenprocoumon, theophylline, warfarin and several others, reducing their effects. “A major problem here is that one often does not know what the exact contents of herbal preparations are, they may not contain what they purport to, and may even contain potent allopathic medicines or adulterants, such as heavy metals,” the president of BPS says in the letter.

In drug–food interactions the food is the precipitant. A heavy meal slows gastric emptying and therefore slows the speed of absorption of many drugs without usually altering the extent of absorption. However, the extent of absorption of lipophilic drugs may be increased, since the presence of food in the stomach gives them more time to go into solution. Some specific foods can also cause interactions.

Drug–device interactions are uncommon. A good example of an interaction in which the device is the precipitant is the risk of angioedema and anaphylactoid reactions to ACE inhibitors (angiotensin-converting enzyme ) in patients undergoing haemodialysis.