For any effective intervention to reach its desired outcome, participants need to take advantage of the advice, treatments and other actions offered by healthcare practitioners. A number of good studies and systematic reviews have evaluated interventions to improve patients' adherence to treatments. One that is used widely is the formation of a contract between healthcare practitioners and participants. We examine here the use of contracts to improve adherence, paying attention to the type of relationship that develops between patients and healthcare practitioners.

Contracts are defined as a mutual agreement between two or more parties that something shall be done by one or both (OED 2003). As a behavioural strategy aiming at improving patients' adherence, contracts refer "to a process of specifying a set of rules regarding some behaviour of interest and formalising a commitment to adhere to them" (Dunbar 1979). They are referred to as simply contracts, behavioural contracts or contingency contracts. Contracts have been used in a wide range of circumstances such as smoking cessation, breast self‐examination, hypertension, diabetes, rheumatic diseases, tuberculosis, hepatitis, in renal patients, and in psychiatric conditions.

The following summarises the features of contracts when used as strategies to increase adherence:

• Formalisation. Contracts are often written, but some examples exist of verbal contracts (Anderson 1982; Arnet 2000).

• Parties. Contracts are most often established between patients and their physicians. There are examples of other parties being involved, such as nurses and patients (Boehm 1997), patients and selected partners from the household or the community (Keane 1984; Lierman 1994; Morisky 2001; Ossip‐Klein 1984), and even contracts with the patients themselves (a self‐commitment made explicit) (Brus 1998). We found one report of a tripartite contract: between the patient, the healthcare practitioner of a Pain Clinic and the Primary Care Physician (Fishman 2002).

• Usually adults. Contracts primarily involve adult patients, although adolescents (Morisky 2001; Wysocki 1989) and children (Greenan‐Fowler 1987; Sherman 1991) have also been involved. The role of children is particularly delicate, since they are often in a triangular relationship with their carers and healthcare practitioners, and they have a special right to access appropriate information (Sanz 2003).

Contracts formalize the commitment of patients and/or healthcare practitioners to follow treatment, prevention or health promotion activities. These usually involve therapeutic activities (particularly adherence to prescribed drugs) but they also include: observance of appointments (Hayes 2000); lifestyle behaviours, such as smoking cessation (Resnikow 1997) and nutrition habits (Boehm 1997); and diagnostic actions, like breast self‐examination (Lierman 1994).

When contracts include a reward conditioned by the accomplishment of the contract clauses, they are referred to as contingency contracts: "a specifically negotiated agreement that provides for the delivery of positive consequences contingent on desirable behaviour" (Janz 1984). There are two main types of rewards (Christiensen‐S. 1985). 'Token economies', which were initially used as a behavioural therapy, are rewards from the healthcare practitioner in the form of tokens that can be exchanged for something of value (Hayes 2000; Wysocki 1989). Rewards may also involve the refund of a deposit ('deposit contract') (Chowdhury 1997; Molteni 1983; Paxton 1983). One study reported a self‐reward, where the patient states what s/he will do to reward him/herself (Neale 1991). Another study involved insurance refund policies based on measures of success of treatments (Harzer 2000). Neither contingencies nor penalties seem to take place, however, if healthcare practitioners do not respect their terms in the contracts.

Ethical issues arise when access to treatment may be dependent upon patients' behaviour as specified in a contract (Biller 1999). Contracts have not been used only as behavioural therapy, but also to support decisions on the appropriateness of a given treatment. For example, one study described how compliance with a behavioural contract was used as a criterion to identify individuals with the potential to maintain a transplanted organ capably (Cupples 2001). The circumstances in which a patient can make a rational and autonomous choice, in the context of contracting, is also worthy of ethical consideration (Biller 1999).

Concordance and the relationship model

Compliance or adherence has been defined as "the extent to which a person's behaviour (in terms of taking medications, following diets or executing lifestyle changes) coincides with medical or health advice" (Haynes 1979a). The increasing use of the term 'adherence' instead of 'compliance' is due to the latter's negative and authoritarian connotations. Adherence implies the patient's active choice in following medical recommendations rather than passive co‐operation in obedience to them (Evangelista 2000). However, adherence still is rooted in a medical model where patients are expected to do what healthcare practitioners tell them. In this review, we use the term adherence in its most restricted sense, to designate the extent to which something that has been implicitly or explicitly agreed between healthcare practitioners and patients, actually happens.

Concordance has emerged as a term that aims to reflect that patients/persons have self‐determination and control over what happens to them. Concordance means shared decision making and arriving at an agreement that respects patients' wishes and beliefs (Jones 2003). Healthcare practitioners may also find, for example, that difficulties experienced by chronically‐ill patients with their treatments may be minimised in the context of a concordant relationship (Townsend 2003).

Some contracts depend on a relationship model. In behavioural contracts, the healthcare practitioner perspective predominates. References to healthcare practitioners' obligations (like providing information or evidence‐based treatments) are generally missing. In the literature scrutinised so far, contracts do not seem to be based on a relationship marked by shared decision‐making, but instead place the responsibility of failing the terms of the contract on the patients' side. The literature around concordance is particularly relevant. With concordance, an essential component in a shared decision‐making model is that of mutual agreement (implicit or explicit) with the treatment decision (Charles 1997). This kind of agreement may indeed reinforce the mutual contribution of healthcare practitioners and patients to a successful treatment (Maher 2003). Furthermore, it has been argued that unless patients and doctors are collectively or jointly involved in the decision‐making process, sharing information and building up consensus, there is no basis for reaching an agreement on which a treatment can be implemented (Stevenson 2000). In a concordant consultation the patient and the healthcare professional participate as partners to reach that agreement (Cox 2002).

Impact on health

Low adherence may seriously compromise the effectiveness of therapeutic regimens. It has been reported that adherence may be as low as 10% in keeping appointments (number of appointments kept in relation to the total number of appointments scheduled) or between 40% and 60% in the case of long‐term medications (percentage of patients with presence of medications in body fluids or self‐assessed reporting of drugs intake) (Sackett 1979). Poor adherence to treatment regimens has been associated with a reduction of the treatment effectiveness, leading to worse health outcomes (Cleemput 2002; Gordis 1979). The WHO report on adherence documents worse outcomes associated with poor adherence for conditions like hypertension, type‐2 diabetes and depression (WHO 2003). Interestingly, however, a Canadian study suggested that adherence to guidelines for the treatment of patients with radiological‐confirmed pneumonia was associated with increased adverse events (Loeb 2001). There is some evidence that the costs involved in treating non‐adherent patients are greater than those involved in treating adherent ones (Cleemput 2002; Heinssen 1995). In the United Kingdom (UK), it was estimated that missed appointments resulted in an economic loss of 250 million pounds sterling per year (DPP 2003).

Advantages of Contracts

What are the potential advantages of contracts over other interventions seeking to improve adherence and concordance? First, they could allow for better replication if they are standardised and do not include extensive training or educational components. Contracts may be cheaper to implement than other combined or more complex interventions or even than supervised self‐administration of drugs (Keane 1984). Apart from that, contracts have shown cost savings related to an increase in adherence and the rationalisation of the care provided (Heinssen 1995). For patients/participants, provided that the interventions used are effective, the benefits include health gains, psychological comfort (Jones 2003) and a better understanding of what they are expected to do and why.

Evidence base for improving adherence

Haynes reviewed the factors associated with the level of adherence to therapeutic regimens (Haynes 1979b). The type of disease seems to play a secondary role, except in specific conditions; for example adherence tends to be lower in some psychiatric disorders such as depression. Organisational issues such as referral delays, waiting times and appointment schedules have a stronger influence on the level of patients' adherence. In relation to the features of the therapeutic regimens, low adherence has been almost constantly found in treatments of longer duration and involving several drugs. Socio‐economic barriers, side effects of treatment and denial of the illness have also been related to poor adherence (Mellins 1992). Finally, the interaction between patients and healthcare practitioners is decisively important in ensuring that what has been explicitly or implicitly agreed, actually takes place. Effective communication of usage instructions for drugs, and the clinician's understanding of patients' concerns about their problems or treatment preferences, have been associated with an increase in patients' adherence and willingness to participate (Hulka 1979).

Interventions to increase adherence may address organisational issues, the simplification of therapeutic regimens, the interface between the patient and the healthcare practitioner and patients' behaviour. Strategies to increase adherence to regimens have been systematically reviewed in general (Haynes 2003) and in relation to specific diseases, like tuberculosis (Volmink 2000; Volmink 2003), HIV/AIDS (Haddad 2003) or mental illness (Reda 2003), reporting the effects on patients' adherence and on other outcomes. These interventions tend to be complex. First, many different actors and activities may be involved. For example, educational interventions may involve physicians, other therapists, facilitators, educational materials, different schedules and structures of the sessions. Second, some interventions are a combination of different strategies (for example, patient instructions with visits to a specialist, or patient brochures with group sessions). This complexity makes it very difficult to know which are the key elements that may have an impact on patients' adherence or on the improvement of health outcomes. Results from these reviews indicate that some strategies or combination of strategies may improve adherence or health outcomes, but their effects are not very remarkable overall when compared with the effort they require (Haynes 2003).

While this review focuses on a single strategy (contracts) in the context of any health condition, several systematic reviews have assessed interventions to improve adherence or compliance in relation to specific conditions. Four of them included contracts. One was restricted to adherence to appointment keeping, and considered only randomised controlled trials written in English (Macharia 1992). Another assessed controlled studies, published in English language journals, of patients' adherence to therapeutic regimes (Roter 1998). Two other reviews were published in The Cochrane Library. One of them focused on tuberculosis (Volmink 2003) and the other considered adherence to prescribed (self‐administered) medications only (Haynes 2003). No systematic review has addressed contracts as a strategy to improve patients' adherence to any kind of treatment, prevention or health promotion activity, regardless of the setting and the condition or disease affecting the patients.