Complex sentence: Local measures such as incision and drainage and removal of the cause of the infection (e.g. a necrotic tooth pulp) have a greater therapeutic benefit and are much more important, and if drainage has been achieved, antibiotics are not usually necessary.
Split into simpler sentences: Local measures such as incision and drainage, and removal of the cause of the infection (such as a necrotic tooth pulp) have a greater therapeutic benefit and are much more important. If drainage has been achieved, antibiotics are not usually necessary.