Many of the most serious patient safety incidents are identified as having a series of errors and failures contributing to the eventual outcome. Safety science proposes models of how these may occur.

One such model is known as the Swiss cheese model of accident causation. This uses the analogy that the slices of Swiss cheese represent different types of safeguard. The holes in each layer of the cheese represent weaknesses in the safeguard, which could be many things, such as an unplanned and uncommunicated change in practice, an IT failure or a design fault.

If multiple slices are lined up in just the right arrangement, there would be a hole all the way from one side to the other, see Figure 1.

When this occurs and an incident is not prevented, it appears that there were no effective safeguards in place.

For example, there may be many safeguards used to ensure that patients are provided with medicines that are safe and appropriate for their use. Some of these safeguards will work prior to a person visiting a pharmacy:

• A patient’s conversation with a GP and a warning that the medicine they are taking is incompatible with other medicines
• Manufacturers’ efforts to ensure the packaging is clear and that the products with similar names or different doses can be easily distinguished
• International and national regulations, policies and recommendations, for example controlled drug regulations.

In the pharmacy, there will be several more safeguards ranging from access to patients’ records, SOPs for dispensing and accuracy checking, and interaction alerts built in to dispensing systems.

All of these could be seen as different layers of the Swiss cheese. However, despite the number of safeguards in place, there is still a probability – a risk – that the holes in the cheese could line up and a patient could be involved in a safety incident.