The invention is directed to forming duplex prodrugs that provide significant increase in the transdermal flux of drugs across human skin. The prodrugs are prepared by reacting a drug that can form an ester moiety with phosgene to form a carbonate. The drug can be an opiate or an opiate antagonist. When in contact with human skin, the duplex drug is biotransformed by enzymes or by hydrolysis into two drug molecules.