A composition comprising: 1) one or more biodegradable block copolymer drug carriers comprising A-B-A or B-A-B block copolymers having a total weight average molecular weight of 2000 to 4990 Daltons, wherein the A block is a biodegradable polyester or poly (ortho ester) and the B block is polyethylene glycol, and the weight percentage of the A block is between 51% to 83% and the weight percentage of the B block is between 17% to 49%; and 2) a polyethylene glycol, a PEG derivative, or a mixture of PEG and a PEG derivative, said PEG or PEG derivative having a molecular weight of 150 to 1100 Daltons, wherein the PEG derivative is an ester or ortho ester derivatized PEG derivatized with D,L-lactide, D-lactide, L-lactide, D,L-lactic acid, D-lactic acid, L-lactic acid, glycolide, glycolic acid, ε-caprolactone, 1,4-dioxan-2-one, ε-hydroxy hexanoic acid, γ-butyrolactone, γ-hydroxy butyric acid, δ-valerolactone, δ-hydroxy valeric acid, hydroxybutyric acids, malic acid, or mixtures thereof, or said PEG derivative is represented by R 1- CO-O- (CH 2 -CH 2 -O) n -CO-R 2 or R 1 -O-(CH 2 -CH 2 -O) n -R 2 wherein R 1 and R 2 are independently H and C 1 to C 10 alkyl and n is an integer between 3 and 20; wherein at least one of the biodegradable block copolymeric drug carriers is soluble in an aqueous solution and miscible with the PEG, PEG derivatives, or mixtures thereof; wherein the weight ratio of the biodegradable block copolymeric drug carrier and the PEG, PEG derivative or mixtures thereof is within the range of 5:1 to 1:99, wherein said composition can be reconstituted in water or an aqueous solution to form a homogeneous solution or a uniform colloidal system within 0.01 minutes to 180 minutes, and wherein the drug carriers possess reverse thermal gelation properties. 1) one or more biodegradable block copolymer drug carriers comprising A-B-A or B-A-B block copolymers having a total weight average molecular weight of 2000 to 4990 Daltons, wherein the A block is a biodegradable polyester or poly (ortho ester) and the B block is polyethylene glycol, and the weight percentage of the A block is between 51% to 83% and the weight percentage of the B block is between 17% to 49%; and 2) a polyethylene glycol, a PEG derivative, or a mixture of PEG and a PEG derivative, said PEG or PEG derivative having a molecular weight of 150 to 1100 Daltons, wherein the PEG derivative is an ester or ortho ester derivatized PEG derivatized with D,L-lactide, D-lactide, L-lactide, D,L-lactic acid, D-lactic acid, L-lactic acid, glycolide, glycolic acid, ε-caprolactone, 1,4-dioxan-2-one, ε-hydroxy hexanoic acid, γ-butyrolactone, γ-hydroxy butyric acid, δ-valerolactone, δ-hydroxy valeric acid, hydroxybutyric acids, malic acid, or mixtures thereof, or said PEG derivative is represented by R 1- CO-O- (CH 2 -CH 2 -O) n -CO-R 2 or R 1 -O-(CH 2 -CH 2 -O) n -R 2 wherein R 1 and R 2 are independently H and C 1 to C 10 alkyl and n is an integer between 3 and 20; wherein at least one of the biodegradable block copolymeric drug carriers is soluble in an aqueous solution and miscible with the PEG, PEG derivatives, or mixtures thereof; wherein the weight ratio of the biodegradable block copolymeric drug carrier and the PEG, PEG derivative or mixtures thereof is within the range of 5:1 to 1:99, wherein said composition can be reconstituted in water or an aqueous solution to form a homogeneous solution or a uniform colloidal system within 0.01 minutes to 180 minutes, and wherein the drug carriers possess reverse thermal gelation properties.