Method of producing a drug delivery system, comprising the steps of: (a) selecting a first drug substance; (b) depositing the first drug substance onto at least one surface region of a medical device so as to form a first deposited drug layer; (c) forming a first gas cluster ion beam in a vacuum chamber; (d) positioning the at least one surface region of the medical device in the vacuum chamber for irradiation by the first gas cluster ion beam; (e) irradiating the first deposited drug layer with the first gas cluster ion beam so as to adhere a drug layer to the at least one surface region of the medical device such that a portion of the deposited drug substance is permitted to be released from the first adhered drug layer at a first expected rate; wherein the adhered drug layer comprises at least one carbonized matrix formed of a portion of the first deposited drug layer and including a plurality of interstices within which is disposed a non-carbonized portion of the first deposited drug layer, a plurality of the interstices being open at a surface of the carbonized matrix so as to permit the non-carbonized portion of the first deposited drug layer to be released at the expected rate; and wherein the at least one adhered drug layer further comprises another non-carbonized portion of the first deposited drug layer that is encapsulated between the carbonized matrix and the at least one surface region of the medical device; (f) selecting an additional drug substance; (g) depositing the additional drug substance as an additional drug layer onto the most recently adhered drug layer; (h) irradiating the additional drug layer with an additional gas cluster ion beam so as to adhere the additional drug layer onto the most recently adhered drug layer such that a portion of the additional deposited drug substance is permitted to be released from the additional adhered drug layer at an additional expected rate; and (i) optionally repeating steps (f) through (i) until a desired number of additional adhered drug layers are formed. (a) selecting a first drug substance; (b) depositing the first drug substance onto at least one surface region of a medical device so as to form a first deposited drug layer; (c) forming a first gas cluster ion beam in a vacuum chamber; (d) positioning the at least one surface region of the medical device in the vacuum chamber for irradiation by the first gas cluster ion beam; (e) irradiating the first deposited drug layer with the first gas cluster ion beam so as to adhere a drug layer to the at least one surface region of the medical device such that a portion of the deposited drug substance is permitted to be released from the first adhered drug layer at a first expected rate; wherein the adhered drug layer comprises at least one carbonized matrix formed of a portion of the first deposited drug layer and including a plurality of interstices within which is disposed a non-carbonized portion of the first deposited drug layer, a plurality of the interstices being open at a surface of the carbonized matrix so as to permit the non-carbonized portion of the first deposited drug layer to be released at the expected rate; and wherein the at least one adhered drug layer further comprises another non-carbonized portion of the first deposited drug layer that is encapsulated between the carbonized matrix and the at least one surface region of the medical device; (f) selecting an additional drug substance; (g) depositing the additional drug substance as an additional drug layer onto the most recently adhered drug layer; (h) irradiating the additional drug layer with an additional gas cluster ion beam so as to adhere the additional drug layer onto the most recently adhered drug layer such that a portion of the additional deposited drug substance is permitted to be released from the additional adhered drug layer at an additional expected rate; and (i) optionally repeating steps (f) through (i) until a desired number of additional adhered drug layers are formed.