Complex sentence: It is a derivative of morphine; to be specific, a hydrogenated ketone thereof, and comparatively, hydromorphone is to morphine as hydrocodone is to codeine and, therefore, a semi-synthetic drug.
Split into simpler sentences: It is a derivative of morphine; to be specific, it is a hydrogenated ketone thereof. Comparatively, hydromorphone is to morphine as hydrocodone is to codeine--that is, a semi-synthetic drug.