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Will the Treatment have Side-Effects?

The ideal medicine would be a "magic bullet". It would target the disease, stop or reverse the underlying process, and never do anything else. Such a medicine does not yet exist. Although many treatments do indeed target the causes of a disease they have additional effects which may not be predictable. All medicines are capable of causing side-effects.

The likelihood of side-effects occurring, however, can vary enormously. It is not usually possible to predict in advance who will experience a particular side-effect. Users should therefore be prepared to accept that they may have side-effects, but they should feel that the expected benefit from the treatment is sufficient to justify the possibility. Whether or not side-effects occur depends on:

• the medicine

• how it is used

• the characteristics of the individual who is taking it

The Medicine

Each medicine is associated with particular types of side-effects. Medicines with similar chemical structures tend to produce similar side-effects. Their occurrence may or may not be predictable (example 1).

Important questions about a particular side-effect with any medicine are:

• What is the chance that it will occur?

• If it does occur, what are the possible consequences? How badly might I be affected?

• Is there anything that can be done to prevent the side-effect or minimise it?

• What action should I take if it occurs?

And taking account of the answers to these questions:

• Is the side-effect acceptable in relation to the expected benefits of the treatment, taking into account the alternatives (including no treatment)?

What is the chance that a side-effect will occur?

Perhaps surprisingly, it is not always possible to answer this question accurately. If a side-effect is common or fairly common, then good information about its frequency is likely to be available. However, a rare side-effect may have only affected a few users and it may be quite uncertain how often it really occurs. A guide to what is meant by common and rare in this context is given in the following table:

For most side-effects it is usually possible to indicate that it falls into one of the above categories or might come into one of two categories (e.g. it seems to be either rare or very rare).

Why should you worry about side-effects that are rare or very rare? The answer is that you only need to be concerned if the consequences are serious. No-one will be too bothered by a 1 in 5000 chance that they may have a headache or feel sick for a few hours. Everyone, however, should be concerned about a 1 in 5000 chance of dying or suffering long-term disability from a medicine used to treat something mild (such as a cold). With such low probabilities it is human nature to expect that it "won't happen to me" but this is not a rational approach. It will happen to somebody and if half a million people use the medicine then 100 people will be affected. And one of them could be you or a member of your family.

What are the possible consequences?

The answer to this question is crucial. It is much easier to picture the effects of some major adverse event on our lives than it is to deal with a 1 in 100 chance that it will happen. As we explain in the next section, all medicines on the market are stringently tested and those which produce serious adverse effects will only be available if they are rare or there are major compensating benefits. Thus, in general, side-effects which are relatively minor (i.e. those that resolve and do not have long-term consequences) occur more often than those that are serious.

A serious side-effect is generally regarded as one which:

• leads to death
• is life-threatening
• may cause or prolong hospitalisation
• causes long-term disability
• produces defects in an unborn child

When a medicine causes serious side-effects this is regarded as a safety issue (see section 2 below). Irrespective of whether a side-effect is serious, it may vary in severity (for example a headache is usually not serious but it can be severe). In dealing with the problem of side-effects doctors need to take account of both seriousness and severity. Only side-effects which are neither severe, nor serious, are likely to be readily acceptable to users.