Multimorbidity is usually defined as when a person has 2 or more long-term health conditions. Measuring the prevalence of multimorbidity is not straightforward because it depends on which conditions are counted. However, all recent studies show that multimorbidity is common, becomes more common as people age, and is more common in people from less affluent areas. Whereas in older people multimorbidity is largely due to higher rates of physical health conditions, in younger people and people from less affluent areas, multimorbidity is often due to a combination of physical and mental health conditions (notably depression).
Multimorbidity matters because it is associated with reduced quality of life, higher mortality, polypharmacy and high treatment burden, higher rates of adverse drug events, and much greater health services use (including unplanned or emergency care). A particular issue for health services and healthcare professionals is that treatment regimens (including non-pharmacological treatments) can easily become very burdensome for people with multimorbidity, and care can become uncoordinated and fragmented. Polypharmacy in people with multimorbidity is often driven by the introduction of multiple medicines intended to prevent future morbidity and mortality. However, the case for using these medicines weakens if life expectancy is reduced by other conditions or frailty. The absolute difference made by each additional medicine may also reduce when people are taking multiple preventive medicines. The implications of multimorbidity for organisation of healthcare are highly variable depending on which conditions a person has. Groups of conditions that have closely related or concordant treatment, such as diabetes, hypertension and angina, pose fewer problems for coordination than conditions needing quite different treatment (for example, physical and mental health conditions).
NICE guidelines have been developed for managing many individual diseases and conditions. The aim of this guideline is to inform patient and clinical decision-making and models of care for people with multimorbidity who would benefit from a tailored approach because of the high impact of their conditions or treatment on their quality of life or functioning. This is a particular concern for generalist medical professionals such as GPs and geriatricians and healthcare professionals such as pharmacists and nurses working in those services; the guideline is also relevant to specialist services because many of the patients they care for will have significant other conditions.
You can also see this guideline in the NICE pathway on multimorbidity.