Complex sentence: The EMT, thinking she is going into shock, administers adrenaline, which stops the transformation-where the "cure" (which turned out to be a sedative) triggered the change, adrenaline has the opposite effect.
Split into simpler sentences: The EMT, thinking she is going into shock, administers adrenaline, which stops the transformation. The "cure" turned out to be a sedative, which triggered the change; and adrenaline has the opposite effect.