Assessing and understanding patient adherence in the management of asthma requires an appreciation of the diversity and complexity of adherence behaviour. Adherence to medication can be defined as the degree to which use of medication by the patient corresponds with the prescribed regimen.

Patients who regularly and consistently follow the prescribed regimen demonstrate adherent use. Adherence to medication is not a dichotomy, however, and patients can demonstrate a wide variety of patterns of medication use. The efficacy of asthma therapies can be modulated by these adherence patterns in several ways.

The most obvious form of nonadherence is chronic under-use, i.e. the patient consistently uses less medication than is prescribed. Chronic under-treatment of asthma may lead to poor control of symptoms and greater reliance on pro re nata (PRN) treatments for the relief of acute asthma symptoms.

Patients may also have an erratic pattern of adherence, in which medication use alternates between fully adherent (usually when symptomatic) and under-use or total non-use (when asymptomatic). Patients with erratic adherence may present for treatment of acute asthma although they apparently adhere completely to their prescribed regimen. Some patients relying solely on inhaled beta-agonists for symptom relief may be prone to over-use during acute bronchospasm. This may cause a patient to delay seeking care, or lead to complications associated with excessive use of beta-agonists (2).

Patients may exhibit a different pattern of adherence to each of the various medications prescribed for the management of their asthma. For example, a patient may under-use the prescribed prophylactic anti-inflammatory ("controller" or "preventer") medications while remaining appropriately adherent to the regular taking of the beta-agonist. Adherence to an asthma action plan that outlines how and when both controller and reliever medications should be taken and when to seek urgent care has been shown to be one of the most effective forms of asthma self-management (3). Finally, in order for medications delivered by metered dose inhaler (MDI) to control asthma optimally, the patient must adhere to the instructions for correct MDI use, or use an MDI spacer. Although MDI adherence has rarely been assessed in clinical or research settings, those studies that have examined patterns of MDI use by patients have suggested that poor technique is widespread (resulting both from inadequate instruction and patients' forgetfulness), and that improved MDI adherence can influence asthma management (4).