Although the interaction between comorbidities and chronic diseases is strong, the effect of comorbidities receives little attention in many chronic diseases. In multiple sclerosis (MS), an increasing amount of evidence suggests that physical and mental comorbidities, and adverse health factors such as smoking and obesity, are common and can affect the disease. These comorbid diseases and lifestyle factors affect clinical phenotype, the diagnostic delay between symptom onset and diagnosis, disability progression, and health-related quality of life. Future studies of comorbidity and MS should consider comorbidities and health behaviours and should take into account the modifying effects of socioeconomic status, ethnic origin, and cultural factors. Studies of the frequency of comorbidities in patients with MS should be population based, incorporating appropriate comparator groups. These studies should expand the range of comorbidities assessed, and examine how the frequency of comorbidities is changing over time. Further research is needed to answer many other questions about comorbidities and their associations with MS, including the best way to measure and analyse comorbidities to understand these associations.
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