In simple terms
Comorbidity describes a second condition present alongside the one being treated. Someone treated for heart failure who also has diabetes has a comorbidity.
The related word multimorbidity drops the idea of a main condition. NICE defines it as the presence of two or more long term health conditions, with none ranked first.
How it works
NICE counts a wide range of things as long term conditions: physical and mental health conditions such as diabetes or schizophrenia, ongoing conditions such as a learning disability, symptom complexes such as frailty or chronic pain, sensory impairment, and alcohol and substance misuse.
Conditions cluster for overlapping reasons. Age is the strongest, and deprivation matters too. In younger people and people from less affluent areas, multimorbidity is often a combination of physical and mental health conditions.
Definitions vary between health systems and studies, which is why prevalence figures differ so widely. The count depends on which conditions a researcher chose to include.
Why it matters
Most recommended treatments are based on guidance from trials whose participants had a single health condition. That advice can fit poorly when several conditions are present at once.
The practical result is treatment burden: more appointments, more tests and more medicines. NICE advises a multimorbidity approach for anyone prescribed 15 or more regular medicines, and says to consider it at 10 to 14.
Where you’ll see it
- Hospital notes and referral letters
- Public health reporting on aging populations
- Eligibility criteria for clinical trials
- Guidance on reviewing a patient’s medicines
Example
A patient attending a clinic for asthma who also lives with depression and osteoarthritis has three long term conditions, and each can affect how the others are managed.
Often confused with
Comorbidity assumes a main condition that the others sit alongside. Multimorbidity treats all the conditions as equal, which is why guidelines increasingly prefer that word.
Key facts
- NICE defines multimorbidity as the presence of two or more long term health conditions.1
- NICE counts physical and mental health conditions, ongoing conditions such as learning disability, symptom complexes such as frailty or chronic pain, sensory impairment, and alcohol and substance misuse.1
- NICE advises a multimorbidity approach for people prescribed 15 or more regular medicines, and says to consider it for those on 10 to 14.1
- About two thirds of people aged over 65 have multimorbidity, which is associated with reduced quality of life and higher mortality.2
- Most recommended treatments come from trials whose participants had a single health condition, which limits how well they fit people with several.2
This entry explains a medical term. It is not medical advice. If you are concerned about your health, speak with a qualified health professional.
Related concepts
Quick checkWhat is the difference between comorbidity and multimorbidity?Show answer
Comorbidity assumes one main condition with others alongside it. Multimorbidity treats all the conditions as equal.
Sources
- National Institute for Health and Care Excellence. Multimorbidity: clinical assessment and management, NICE guideline NG56. 21 September 2016 (accessed 22 September 2026)
- British Journal of General Practice. Clinical assessment and management of multimorbidity: NICE guideline. May 2017 (accessed 22 September 2026)
Editorially reviewed by Specialty Digest Editorial TeamLast reviewed September 22, 2026Researched and drafted with AI assistanceReport an issue