The most common side effects of ZYNTEGLO on the day of treatment are increased heart rate and abdominal pain. The most common side effects of ZYNTEGLO following treatment for up to 6 months are low level of platelets, which may reduce the ability of blood to clot and may cause bleeding, low level of white blood cells, which may make you more susceptible to infection, and pain in arms or legs. It is also important to note that these may not be all of the side effects you may experience.