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