A biocompatible gel-forming drug-delivering composition, comprising: a hydrophobic drug having a water solubility below 10mg/ml; a first component comprising at least one sulfhydryl group-containing compound in a liquid medium having an alkaline pH, wherein said sulfhydryl group-containing compound is given by the formula Compound, - m, wherein m≥2; and a second component comprising at least one sulfhydryl reactive group-containing compound in either a liquid medium having a neutral or acidic pH or in powder form, wherein said sulfhydryl reactive group-containing compound is given by the formula Compound 2 -Y n, wherein Y is a sulfhydryl reactive group and wherein n≥2; wherein at least one of the first or second components is a polyalkylene oxide and wherein the sulfhydryl groups and the sulfhydryl reactive groups react with one another to form covalent bonds therebetween when said components are mixed together to form a gel in less than one minute, for use in a method for therapeutic treating of tissues by in vivo administration, comprising the steps of: administering to a tissue site the first component; simultaneously or subsequently administering to the tissue site the second component; and subsequently administering to the tissue site the drug; and allowing the sulfhydryl groups and the sulfhydryl reactive groups to react with one another to form covalent bonds therebetween to form a sterile gel in less than one minute a hydrophobic drug having a water solubility below 10mg/ml; a first component comprising at least one sulfhydryl group-containing compound in a liquid medium having an alkaline pH, wherein said sulfhydryl group-containing compound is given by the formula Compound, - m, wherein m≥2; and a second component comprising at least one sulfhydryl reactive group-containing compound in either a liquid medium having a neutral or acidic pH or in powder form, wherein said sulfhydryl reactive group-containing compound is given by the formula Compound 2 -Y n, wherein Y is a sulfhydryl reactive group and wherein n≥2; wherein at least one of the first or second components is a polyalkylene oxide and wherein the sulfhydryl groups and the sulfhydryl reactive groups react with one another to form covalent bonds therebetween when said components are mixed together to form a gel in less than one minute, for use in a method for therapeutic treating of tissues by in vivo administration, comprising the steps of: administering to a tissue site the first component; simultaneously or subsequently administering to the tissue site the second component; and subsequently administering to the tissue site the drug; and allowing the sulfhydryl groups and the sulfhydryl reactive groups to react with one another to form covalent bonds therebetween to form a sterile gel in less than one minute administering to a tissue site the first component; simultaneously or subsequently administering to the tissue site the second component; and subsequently administering to the tissue site the drug; and allowing the sulfhydryl groups and the sulfhydryl reactive groups to react with one another to form covalent bonds therebetween to form a sterile gel in less than one minute