I need more information to decide its usefulness for my son. The drug might not be available here, unless private funding for research is adequate. Your question would be answered best by one of the deflazacort researchers who has current information. They cause blood clots to form in the heart sometimes. These clots can pop out, go to the brain in the arteries, and cause a stroke.
Mitochondrial diseases can sometimes increase the risk of a stroke.
But it potentially can have some ill effects. There are some reports of FSH causing disturbances of the heart's conduction system. Arrythmias called atrial flutter and atrial fibrillation are the most likely problems to occur. Atrial fibrillation can be associated with stroke from embolism from the heart, fainting spells, chest pain, and potentially sudden death. All that said, serious problems are probably uncommon.