Prescribe professionally and safely

Managing drug interactions is an important part of the prescriber's role. The multidisciplinary team, including clinical pharmacy services, have opportunities to assist in the identification and management of drug interactions in secondary care.

In contrast, prescribers in primary care face greater challenges because they alone will be ideally placed to identify and make decisions with regard to drug interactions.

While community pharmacists are able to identify drug interactions, patient choice as to where prescriptions are dispensed and the lack of comprehensive universal patient records mean that community pharmacies do not always have the information needed to spot interactions.

The incidence of drug interactions is difficult to measure accurately and their place in iatrogenic disease difficult to analyse. Reports on incidence vary depending on factors such as physical location of prescribing and medication administration (for example, critical care, inpatient and outpatient settings), context of practice (primary or secondary care) and data acquisition and reporting mechanisms in the studies.

There are several challenges that prescribers face when identifying and managing drug interactions, such as which source(s) is/are best for checking drug interactions, how accurate these resources are and to what extent they are evidence-based.

Resources
There are several reliable sources for checking drug interactions, including drugs directories such as MIMS and the BNF, the drug manufacturer's summary of product characteristics, guidance from individual faculties/royal colleges and specialist texts, such as Stockley's Drug Interactions. While all these sources are accurate and up to date, they are all incomplete.

For many drugs there is a shortage of robust evidence on drug-drug interactions because the data are usually obtained from sources such as pharmacovigilance reporting mechanisms and case reports.

In addition, there is less information available to the prescriber on new medicinal products and their interactions with the plethora of medicines prescribed.

The ability to identify known drug-drug interactions is aided by reliable texts documenting the interactions in a systematic manner.

Greater challenges are faced when identifying interactions between conventional medications and herbal products, because of a relative lack of regulation in the manufacture of herbal products.

Matters that are an inherent part of medication licensing, such as safety, quality and efficacy, will not necessarily occur in unlicensed products and subsequently, the accurate quantification of interactions will be impossible.

The paucity of information available to inform prescribing decision-making in the context of interactions between prescribed medication and OTC medication, foods and food supplements, smoking and recreational drugs is also an important consideration when adding or removing medication from a patient's prescription.

Clinical significance
The clinical significance of drug interactions is difficult to predict in a given individual because there is a wide inter-individual response to drugs.

In the context of drug interactions it appears that a variation in pharmacokinetic parameters has the greatest influence on the potential for drug interactions to occur. Our greater understanding of pharmacogenetics and pharmacogenomics may contribute significantly to knowledge of serious drug interactions.

The BNF has a 'black dot' identifier for 'potentially serious' interactions and specific guidance is explicit enough to assist the prescriber in sound patient-oriented decision-making. However, in practice, combinations of 'potentially serious' interacting drugs are used and there also exists the possibility that drugs may interact despite there being a lack of evidence.

In each of these cases the context of the prescribing is important. The prescribing of an interacting combination of cardiovascular agents may be appropriate by a specialist with multidisciplinary team support and a range of monitoring equipment to hand. The same combination may prove much more hazardous in other contexts of practice.

Managing interactions
While drugs directories are an accurate source of information for prescribing purposes, in that they attempt to identify drug interactions that are potentially hazardous (or otherwise), the text offers little in the way of management of drug interactions. Stockley's Drug Interactions also gives general practical advice on managing certain interactions and the likelihood that the interactions may have consequences for patients.

More comprehensive guidance can also be found. A recent publication entitled Drug interactions with hormonal contraception by the Faculty of Sexual and Reproductive Healthcare gives comprehensive guidance on management of drug interactions with hormonal contraception.

Prescribing decision-making relies on factors such as experience, knowledge and professional judgment. Good decision-making in the evaluation of drug interactions has its roots in many knowledge domains but illustrates the importance of the science (pharmacological understanding) and art (accurate history taking) of prescribing.

When GPs take on the role of supporting and supervising non-medical prescribers, they may be asked for practical guidance as to the management of drug interactions in practice. The National Prescribing Centre prescribing competency entitled Clinical and pharmaceutical knowledge incorporates understanding drug interactions. In order to maintain and develop prescribing competency, drug interaction management should form part of the prescriber's CPD.

• Dr Young is a principal lecturer and prescribing lead, University of Glamorgan. He is also the award leader for the University of Glamorgan and Royal Society of Medicine MSc in 'independent prescribing'

Reflect on this article and add notes to your CPD Organiser on MIMS Learning

Resources

• Walker R, Whittlesea C. Clinical pharmacy and therapeutics (fourth edition). Section 1 pp 40-51. Oxford, Churchill Livingstone, 2007.