Adolescence is commonly defined as the age range between 11 and 19 years. It is a time of significant growth and development [11]. The successful transition from childhood to adulthood demands the achievement of a number of developmental tasks (Fig. 22.2). The accomplishment of these milestones allows the adolescent to individuate from the family and to gain progressively more control and independence. This progression, however, may not only involve tensions but also hinder medical adherence as the adolescent struggles with emerging autonomy. Responsibility for medical decision-making and management shifts from parent to patient, and the tasks of adolescent development may take precedence over other demands such as medication regimens, clinic visits, or lifestyle decisions. Research into chronic diseases such as inflammatory bowel disease, diabetes, HIV, and cystic fibrosis has found that adolescents are a group particularly at risk for poor adherence [14–17].

Recent literature advocates considering adolescence from a biopsychosocial approach, recognizing that adolescence involves all three of these components. Rather than considering adolescence from a discrete, progressive “stage” perspective, it is important to acknowledge that the adolescent exists within a dynamic system. This system is influenced by both internal (physical and psychological) and external (social) demands. Negotiation of any one developmental task of adolescence may influence and also be reliant on the accomplishment of other tasks at the appropriate time [12]. The introduction of a medical condition into the life of an adolescent may perturb this complex developmental system, with implications both for the adolescent’s development and for the medical condition.

In studies of perceived barriers to adherence among adolescents with chronic illnesses, five themes are identified most commonly. These are (1) implications for physical well-being, (2) forgetting due to distraction or lack of planning (e.g., missing doses due to engagement in another activity or not carrying medications when not at home), (3) striving for normality, (4) relationships with peers, and (5) relationships with parents [18]. Additional perceived barriers include treatment perceptions of patients (e.g., lack of belief in treatment effectiveness or usefulness), regime complexity (e.g., number of drugs, number of doses, timing of doses, clarity on medication administration instructions), challenges with organization (e.g., planning when to take medication), and financial costs [18]. Of these themes, the notion of a desire for normality, relationships with peers, and relationships with parents are issues specific to the adolescent population that have not been identified as challenges within the literature on adherence among adults.

New expectations and demands emerge as the adolescent transitions to adulthood. The definition of “young adult” varies in the literature; in North America, the term is traditionally applied to those between 18 and 29 years of age. This is a subpopulation which has been acknowledged only recently as distinct from the broader “adult” demographic and which is accordingly less well studied. Use of the term “emerging adult” may be helpful when considering adherence factors among this group. This terminology acknowledges that individuals in early adulthood continue to experience and participate in developmental changes through which an identity, worldview, and enduring approach to problem solving are shaped [19]. Tasks that are encountered during this life stage include adaptation to newly emerging intellectual abilities, balancing of peer and family influences, adjustment to society’s behavioral expectations, transition away from parental involvement, increasing financial responsibilities, internalization of a personal value system, exploration of sexuality, and preparation for the workplace [10, 20]. For those with chronic conditions that persist from adolescence to young adulthood, such as cystic fibrosis and asthma, the time of transition from the pediatric to the adult healthcare system represents a window during which the risk of compromised adherence is particularly high [5, 20, 21].

With the advent of more successful treatment for cancers in childhood and adolescence, the role of adherence has gained greater importance because therapy is almost always given with curative rather than palliative intent. The emergence of more oral anticancer drugs similarly demands a greater attention to facilitating adherence as greater responsibility falls on the patient [25]. The clinical implications of poor drug adherence are enormous since strict adherence to chemotherapeutic protocols is essential to secure optimal outcome. Nonadherence with oral chemotherapy may play a role in the long-term prognosis of childhood leukemia [10, 14], in the relapse rate [15–17], and in graft survival after transplantation [12]. In chronic myeloid leukemia (CML), adherence to tyrosine kinase inhibitor (TKI) therapy has been associated directly with molecular response. As TKI therapy is a lifelong intervention in AYAs newly diagnosed with CML, poor adherence may have significant implications for survival [20]. In clinical trials, nonadherence can impact negatively on evaluation of trial aims and goals and may lead to an overestimation of required dosage and resultant significant toxicity and morbidity because of perceived lack of response.

Drug nonadherence may obscure the actual rate of adverse reactions and may lead to a waste of resources. The availability of venous access ports and easy-to-operate pumps makes the administration of parenteral chemotherapy at home (“home care”) possible, but this introduces a new dimension to the issue of nonadherence. Examples of consequences of nonadherence in AYA patients with cancer are captured in Fig. 22.3.

The rate of nonadherence among AYA patients with cancer is not well known, due to a lack of large-scale, population-specific studies. Furthermore, interpretation of existing studies is complicated by variation in the definition of nonadherence [25]. Among studies in which the AYA group is either the entire sample or a part of the sample, nonadherence rates have ranged from 27 % to 63 % [5, 10, 26]. Adolescents are less adherent than younger or older patients with cancer, even when treated with similar protocols [10]. Adherence in clinical trials is inferior among the adolescent population, affecting the ability to research new interventions that may improve outcomes [22]. When examining rates of treatment refusal, findings vary quite broadly. A study with a large cohort of 576 Australian patients with various cancers, including both solid and hematologic malignancies, found only 1 % of AYAs failed to complete planned therapy due to nonadherence [11]. An older study found that 23 % of adolescent patients refused all or parts of treatment; of this group, 12 % refused all therapy, 35 % refused one treatment modality when multimodal treatment had been recommended, and 24 % started but failed to complete therapy [27].

Approaching adherence from a strength-based orientation may prove beneficial since the focus is on promoting success rather than anticipating failure. Strength-based models have a strong evidence base in the adolescent medicine literature [28–30]. Factors that have been found to improve adherence among adolescents with cancer include a positive family relationship with open communication, good patient–provider communication, involvement of the adolescent in treatment decisions and illness management, and supporting continued participation of adolescents in usual activities, including social activities and “rite-of-passage” events such as school formals and graduations [10]. These factors are extrapolated commonly to be relevant to young adults as well.

Just as there are differences in the biology and characteristics of malignancies in the AYA population, so too do the behaviors and needs of this demographic differ from other age groups confronting cancer. In order to understand the mechanisms leading to lack of adherence among adolescents, clinicians must recognize that adolescent brain development involves early predominance of the limbic system (i.e., the reward system) and only later the maturing of the prefrontal cortex (i.e., executive functioning and planning) to mitigate the impulses of the limbic system. Thus, the adolescent with cancer may prioritize immediate rather than future experiences disproportionately and may be more focused on the present and on surviving the current treatment than on future and long-term outcomes [31]. A review of general barriers to adherence has been addressed in a previous section. Factors that influence adherence specifically among AYAs with cancer are similar to those observed among AYAs with chronic illnesses in general. These include patient emotional functioning (depression and self-esteem), patient health beliefs (perceived illness severity and vulnerability), and family environment (parental support and parent–child concordance) [26]. While these barriers seem to associate more closely with characteristics and behaviors of the individual than the provider or the treatment, adherence challenges may also center around the healthcare system as it often is not oriented toward the unique needs of the AYA group [32]. Of note, while there is a body of literature exploring adherence among adolescents with cancer and among AYAs with other chronic illnesses, the relatively recent acknowledgment of young adults with cancer as a unique population leads to a dearth of significant literature examining the young adult population. Barriers to adherence among adolescents may be assumed to persist for young adults; however, the limitations of this assumption must be acknowledged.