Box 1. Barriers reported in the included studies.

Barriers	Customer level	Pharmacy staff level	Pharmacy level	System level
Patient/customer reported	Lack of time 28,52 Lack of interest 28 Low symptom awareness and recognition 72 Reluctance to discuss symptoms 72	Lack of awareness of pharmacy staff’s role in early cancer detection 53,72 Age and gender of counter staff 72 Capacity of staff 53	Lack of privacy 25,72 Variation in the desired level of pharmacy input 25 Pharmacy too busy 72 Low awareness of services offered in pharmacies 53
Pharmacy staff reported	Fatalism by patients 27 Forgetting to do the test 27 Restricted eligibility criteria 27,54,61 Lengthy process of intervention 43 Lack of time 61 Having been affected by cancer 61 Reluctance to discuss symptoms 61 Clarity on project 54 Urgent referrals causing distress to patients 55	Pharmacist- only recruitment 27 Time-consuming project training 41,43 Lack of time 43,51,61,62 Change in staffing 61 Lack of staff 73 Locums are unable to refer 73 Frequent use of locums 51 Knowledge gap 51 Limited role 55 Limited scope 55 Fatigue from repeat patient presentations 55 ‘Double-edge sword‘ of knowing the patient 55	Disruption in workflow 28,41 Lack of compensation 28 Competing initiatives 61 Accessing training information challenging 73 Accessing good-quality information time consuming 73 Engagement with large pharmacy chains 73 Unavailability of consultation space 62 Nature of over-the-counter consults 51 Project set-up 54	Unsuitability of test 27 Lack of project advertising 27 Customers do not understand the project 61 Poor communication between pharmacies and other healthcare settings 51,54,73 Lack of referral pathways 51 Lack of integrated data flow 54 Pressures on the healthcare system 55
Other stakeholder reported				Over-screening 23 Workflow ownership 23 Care coordination 23