Non-adherence can be thought of as two related types of behaviour:

Unintentional – when patients intentions to take the medication as advised are thwarted by barriers that are essentially beyond their control, such as forgetfulness, poor comprehension language barriers, or physical inability to manage the medication (e.g. difficulties opening containers or using administration devices).

Intentional – a conscious decision by the patient to take the medication in a way which differs from instructions, or not to take it all. This often takes the form of patients reducing the frequency of dosing or the number of medications down to a level that they (and not their doctor) feel is appropriate, or premature discontinuation of therapy.