The synthesis and use of bifunctional molecules to improve the topical and transdermal delivery efficiency of various types of therapeutic agents or agents designed to promote the transdermal delivery of those therapeutic agents either into the skin tissue or into the systemic circulation. Three major classes of molecules are covalently joined as bifunctional substances; chemical vasodilators, passive dermal penetration enhancers and therapeutic or diagnostic drugs. Chemical vasodilators may be delivered into the skin to increasing the blood flow in a tissue that has compromised circulation or they may be used as part of a delivery vehicle to promote the delivery of the drug. Passive dermal penetration enhancers are those chemicals that promote the passive penetration of drugs and other chemicals through the stratum corneum and epidermis of the skin tissue. Drugs and diagnostic agents are the third group of chemicals that are candidates for the linkage of molecules.