{HEAD ~ A useful therapeutic or prophylactic concentration} will vary from condition to {TAIL ~ condition} and in certain instances may vary with the severity of the condition being treated and the patients susceptibility to treatment 
vary to
{ HEAD ~ A useful therapeutic or prophylactic concentration } will vary from condition to { TAIL ~ condition } and in certain instances may vary with the severity of the condition being treated and the patients susceptibility to treatment
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL OTH OTH REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH