Expert consensus of the Chinese Association for the Study of Pain on pain treatment with the transdermal patch

Table 2 Factors influencing drug percutaneous absorption[10,12]

Influencing factor	Effect on transdermal drug absorption
Drug concentration	Generally, the amount of drug absorbed per unit area per unit time increases with the increase in TDDS drug concentration
Drug distribution coefficient	Drugs with both water-soluble and fat-soluble properties can be effectively absorbed through the skin. The water-soluble properties of drugs determine the concentration of the drug at the absorption site and the partition coefficient affects the rate of drug transport at the absorption site
Drug molecular weight	The ideal relative molecular weight for transdermal administration is 400 Da or less
Carrier factor	The main effects of carriers on percutaneous absorption include solubility of drugs in carriers and change of drug distribution coefficient by carrier
Site of application and time	The larger the application area (TDDS) and the longer the application time, the more the drugs are absorbed
Skin conditions	Hydration of skin helps increase percutaneous absorption. TDDS can form a closed water barrier with evaporating sweat to increase the hydration degree of the skin. It can be applied to the thin cuticle, with better absorption through the skin. When the skin is damaged, the drug will directly enter the subcutaneous tissue and capillaries, which may affect the properties of TDDS

TDDS: Transdermal drug delivery system.