An interaction checker answers only the first question: might an interaction exist? Patient care depends on what follows — clinical significance, mechanism, evidence, monitoring, dose adjustment, duration and alternatives. A defensible workflow layers rapid screening, the exact products’ SmPCs, SPS or other practical guidance, detailed evidence resources such as Stockley’s, maintained specialist checkers and, when uncertainty remains, pharmacy or medicines-information support.
Which source for which question
Rapid screening: use a current, validated interaction checker in the clinical system or medicines platform. eMC section 4.5: read the authorised interaction information for each exact product. SPS: use practical management advice where an applicable topic exists. Stockley’s: use detailed evidence, mechanism, clinical significance and management options. Specialist checkers: use them only within their maintained scope, such as HIV, viral hepatitis, QT risk, reproductive health or oncology. Pharmacy or a medicines-information service: escalate unresolved or high-risk cases with the patient’s context attached.
Worked example: apixaban
The DOAC interactions cluster illustrates why mechanism matters more than lists. Strong CYP3A4 and P-glycoprotein inhibitors raise apixaban exposure, a pharmacokinetic problem pointing one direction; strong inducers lower it, the opposite hazard, reduced anticoagulation dressed as tolerance; antiplatelets, NSAIDs and serotonergic antidepressants raise bleeding risk pharmacodynamically without touching drug levels at all. Three different mechanisms, three different management logics, and a checker that flags all of them equally has told you almost nothing; the apixaban clinical page frames the classes with links to SPS's DOAC interaction guidance and the product SmPCs, which is the orientation step before Stockley's or pharmacy settles the specific pair.
Worked example: amoxicillin
Common antibiotics carry a compact interaction set worth knowing by shape: methotrexate, where the combination can raise methotrexate toxicity risk and warrants respect; warfarin, where antibiotics are a classic destabiliser of INR and a prompt for closer checking; allopurinol, associated with more frequent rash; and probenecid, which raises amoxicillin levels by design, historically deliberately. None of these is exotic, all are screenable in the first-pass layer, and the amoxicillin page carries the orientation with its sources; the value of knowing the shapes is speed, the screen confirms what you already suspected rather than surprising you.
The pitfalls that cause real errors
Four, repeatedly observed. Searching only prescription medicines: over-the-counter NSAIDs, herbal products, St John's wort above all, sit outside many patients' medication lists and inside many serious interactions. Treating every listed interaction as equal: lists are sensitivity-optimised; your job is specificity, which is what the mechanism-and-evidence layer exists for. Ignoring context: dose, duration, renal function and timing modulate almost every interaction, and a static pair-check cannot see them. And leaning on an unmaintained specialist checker: a stale interaction database is worse than none, because it answers confidently; check the maintenance status of any specialist tool before trusting it as sole source.
The workflow, assembled
Screen with a current validated interaction checker; inspect section 4.5 of the exact products’ SmPCs; use SPS for applicable practical advice and Stockley’s for detailed evidence and management options; use specialist checkers only within their defined scope; and refer unresolved or high-risk cases to pharmacy or a medicines-information service with renal function, duration, indication and the findings already gathered.
Frequently asked questions
Is there a good free UK interaction checker for patients?
The NHS medicines pages flag key interactions readably, and patients should be actively told to declare over-the-counter and herbal products; patient-grade tools screen, they do not manage, and the pharmacist remains the accessible expert.
How should multi-drug regimens be approached?
Screen the whole list, then think in mechanisms rather than pairs, additive bleeding risk, additive QT load, competing metabolism, which is where polypharmacy interactions actually live; structured medication review is the proper container for it.
When is stopping the interacting drug wrong?
Often: many flagged combinations are managed with monitoring, timing separation or temporary adjustment, and reflexive stopping trades a managed risk for an unmanaged one. The management layer exists precisely because avoid is only one of its verbs.