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Dimethyl fumarate sometimes causes ‘flushing’ (an unpleasant but not dangerous experience of redness and warmth especially in the face, sometimes associated with itching) and tummy upset. Blood test abnormalities are possible and you will be monitored for these. There is also the unlikely possibility of a severe reaction to any study drug. The effects on pregnant women and breast-fed babies are uncertain so such women should not participate in this part of the study.