Health workers need to share their understanding and perceptions of benefits and harms of treatments with patients and their families as fully as possible. When doing so it is important to remember that how we personally value particular benefits and harms may well differ from how another person values them. A clinician who recommends an intervention does so believing that its benefits outweigh the harms that it can cause. In most consultations, time is too short to explain in detail what these benefits and harms are, or to find out what the patient thinks about them. Moreover, most clinicians are not practised at describing and explaining benefits and harms clearly to patients, and often they also lack important information about these aspects. Very often people use the word.risk. when they mean “harm”, and this causes ambiguities and confusion. The widely used expression.benefit/risk ratio. is meaningless - no such ratio exists. Before a decision is made to use an intervention, its benefits and harms must be weighed, ideally by the clinician and the patient together. Other advantages and disadvantages, such as convenience and cost, may also be relevant. This analysis requires use of the same dimensions for considering both benefits and harms. These dimensions are obvious, but have not been generally recognised. In this context any benefit or harm has four dimensions:¹

1. Its nature, described by its quality, its intensity, and its time course (onset, duration and reversibility).

2. The probability that it will occur.

3. Its importance to the person experiencing it.

4. How the benefit can be maximised or the harm prevented or minimised.

The clinician is expected to know or find out about the nature and probability of each benefit and harm, and how to maximise benefits and minimise harms. But only patients can say how they regard the hoped for benefits and the possible harms, though many need help to think clearly about them. Clinicians should identify how much the benefits matter to their patient - for example, are the benefits of taking a medicine or having an operation “worth the trouble”? - and whether a specific harm is particularly threatening or would be intolerable to that particular patient. People's fears, wishes and priorities differ greatly and unpredictably.