As the Swiss cheese model shows, there may be many factors that interact and lead to a failure. All of these factors should be explored as it may not be the last safeguard to fail that is the most significant.

Another analogy describes the weaknesses or ‘holes in the cheese’ as ‘resident pathogens’, which are present in the system but may not necessarily lead to an accident just because they are present. These are often described as ‘latent failures’.

An example of a ‘resident pathogen’ in a pharmacy dispensary is medicines with similar names or packaging – these are almost always present but do not always lead to a dispenser confusing them and choosing the wrong package from the shelf.

Consider the formulation mismatch scenario discussed last month with the prescription for prednisolone 5mg entericcoated tablets:

Rabia is a locum pharmacist on duty at a busy community pharmacy. It is lunchtime and she is the only person in the dispensary. There are several people waiting to be served.Rabia is handed a prescription for prednisolone 5mg entericcoated tablets which she dispenses and checks herself. She does not realise that she has taken prednisolone 5mg off the shelf instead of the entericcoated version that is stored next to it, and she supplies this to the patient.

The system is designed so that the safeguard is the pharmacist and their actions – but could other factors contribute? In this scenario, the holes in the Swiss cheese have lined up. Even though the pharmacist was the last barrier before the patient, there may be other factors that contribute, such as the storage of the medicines, the packaging style chosen by manufacturers or the working environment.