The possibility that hepatitis B vaccine may cause or exacerbate multiple sclerosis stems from several case reports of onset or recurrence of symptoms of CNS demyelination shortly following vaccination. It is difficult, however, to infer causation from individual case reports since they may simply represent coincidental temporal associations with vaccination. There is only weak, nonspecific evidence to support the biological plausibility of an association between hepatitis B vaccine and multiple sclerosis. Epidemiological studies have found that hepatitis B vaccine does not increase the risk of developing multiple sclerosis or cause exacerbations. The US Institute of Medicine and other review panels have concluded that the evidence favors rejection of a causal association between hepatitis B vaccine and multiple sclerosis.