Prescribing experts Professor Tony Avery and Dr Reya Shah discuss five key challenges in the management of potential drug interactions when prescribing in general practice.
Learning objectives
This module will support your understanding of drug interactions and how to manage them, in particular with regard to:
- The most useful resources and alerts for drug interactions, and how to best use them in practice
- How to navigate antibiotic prescribing in patients on methotrexate
- The risks associated with NSAIDs in patients on oral anticoagulants
- How the rise in polypharmacy is increasing the likelihood of drug interactions, including some lesser known examples
- Over-the-counter drugs, foods and supplements with potential to interact with prescribed medicines
1. Therapeutics is such a complex area that we routinely rely on external sources and warning systems to alert us to possible interactions, but these sources may give varied or conflicting advice – what are the most trusted and user-friendly resources in this area?
The main reason for variation in the information on drug interactions is that the evidence for possible interactions varies considerably. At one end of the spectrum there is strong epidemiological and pharmacological evidence, while at the other there are anecdotal reports and theoretical (but unproven) reasons why an interaction may exist. The different sources of information have different ‘cut-offs’ for what they display to clinicians, leading to considerable variation. It doesn’t help that most of the sources of information available to GPs provide little additional information apart from the interaction itself and the likely level of associated hazard; this makes interpretation difficult.
The most comprehensive interaction checker from our experience is Stockley’s.1 This has around 4,500 detailed monographs of potential interactions: describing the purported mechanism; the expected effects; and the strength of evidence. Stockley’s is available as a book, and online through Medicines Complete. Some hospital trusts, ICBs and PCNs have access to this product, so it may be worth checking locally. Otherwise, the NHS Specialist Pharmacy Service (SPS) can provide advice on individual queries from GPs, through its Medicines Advice page. The SPS website also has guidance on notable or important interactions.2
The British National Formulary (BNF) is a reliable source of information on drug interactions but does not contain a lot of detail. It is freely available to all NHS staff and its interaction checker is particularly useful via the BNF app, in which you may submit a full list of medications and receive a list of potential interactions colour-coded according to the strength of evidence behind these recommendations and the severity of the potential interaction.
GPs will also have information on potential drug interactions flagged up by their clinical computer systems. EMIS draws upon the BNF for its interaction alerts, and TPP SystmOne uses the Multilex database. GPs, like many other clinicians, often complain of being ‘over alerted’ by their clinical computer systems, but it is worth checking these alerts before prescribing, particularly those that are flagged of high(er) importance.
In addition to interaction alerts that are embedded in GP computer systems, there are also clinical decision support tools that can work alongside these systems and provide additional information and guidance for specific hazards; examples include OptimiseRx and ScriptSwitch. These products do not provide comprehensive interaction alerts, but OptimiseRx has several important alerts, and both provide alerts and advice to support medicines optimisation more generally.
Advice is also available for specific types of interaction. For example, there are several scales available for assessing anticholinergic burden.3 One of these, the ACB scale has been incorporated into the main GP clinical systems so that it is possible to calculate ACB scores for patients when doing medication reviews, or to interrogate the system to identify patients with high ACB scores for further assessment. Interaction checkers are also available for hepatology drugs, HIV drugs and Covid-19 drugs.4
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Dr Tony Avery is a GP and Professor of Primary Health Care at the University of Nottingham, and Dr Reya Shah is Specialty Registrar in Clinical Pharmacology and Therapeutics, St Georges University Hospitals NHS Foundation Trust, London
