Interactions

Last updated: February 07, 2020

Key points

  • Information sources and e-prescribing systems may flag up a range of potential interactions but:
  •                  – Some interactions are common, whereas others are uncommon or theoretical. 
                     – The severity of interactions varies between patients: people aren't affected equally. 
  • Depending on the severity and nature of the interaction, options for management could include stopping or changing one drug; reducing the dose of one drug; continuing the combination and monitoring the patient.
  • Some medicines are particularly likely to interact with lots of other medicines such as warfarin, ciclosporin, phenytoin, carbamazepine, fluconazole, and ciprofloxacin. It's good to know these and other common culprits. 

Info sources

✦  The SmPCs for medicines on the eMC site describe their Interactions (section 4.5). Sometimes there is extra info in the Contra-indications, Special Precautions, and Undesirable Effects sections too.
✦  BNF online gives basic info about many interactions. Type the name of the medicine then scroll down the page to the Interactions section.
✦  The websites Drugs.com and Medscape both have free drug interactions checkers.
✦  There are specialist websites for interactions involving cancer medicines, HIV medicines, and viral hepatitis medicines provided by the University of Liverpool. There is also a website for antifungal medicines interactions.

Asking the right questions

When tackling any clinical problem you may need to find out about the patient's medical history. However, here are some questions that you may want to ask the patient or a healthcare professional specifically in connection with an interactions-related clinical problem:
    THE MEDICINE:
  • Which drugs are involved, and is the patient already taking both of them? Commonly, a patient already taking one medicine is prescribed a second one which may interact. In practice it’s often easier to change the new drug if you can.
  • What is the indication for the medicines involved? This enables you to advise on alternative treatment if necessary, and sometimes the indication will affect the remedial action or monitoring required. 
  • How long have they been taken for?  If a patient has been stable on a combination for a long time without ill effects there may be no need to change the prescription.
  • If there is an interaction, is there any reason why alternative drugs can’t be used to avoid it? Sometimes alternatives that are less likely to interact have already been prescribed or were not tolerated. 

THE PATIENT
  • If the patient is already taking both drugs, have any problems been identified or investigated?  You need to know if the combination has already made the patient ill, or if the prescriber is worried about a particular aspect.
  • Is the patient taking any other drugs?  It’s important to have the full picture to avoid potential interactions that may have been missed.  
  • What is the patient’s liver and renal function like?  This can affect clearance which may make the patient more prone to interactions or adverse effects.
  • Has a healthcare professional or patient read a warning about the combination? Sometimes e-prescribing systems or websites flag up interactions indiscriminately; they are not always relevant.

GOING FORWARD:
  • If any monitoring would be required, who would do this? If your advice is to continue with a potentially interacting combination, or to change a medicine, you need to ensure the patient will be monitored by e.g. blood tests, checking efficacy, looking out for side effects. Who will do this? You should talk to the patient about this too.
  • Who needs to know? Make sure that any problems related to interactions are clearly communicated to the patient and the patient's GP, so that any corrective actions you take or recommend are understood. 


Our partner site the Medicines Learning Portal has e-learning about drug interactions. It was written as an introduction for new hospital pharmacists but is likely to be helpful to primary care professionals too. You should allow about an hour to complete it.