A topical drug can be absorbed and have systemic effects. Sometimes this is the desired effect and sometimes not. Systemic absorption of a topical drug is the basis of transdermal medications.
Example of a topical agent where systemic absorption is desired: testosterone cream
Example of a topical agent where systemic absorption is undesirable: topical antiseptics, topical corticosteroids
Newborns are particularly at risk.
Factors favoring drug absorption:
(1) decrease in skin integrity
(2) larger application area
(3) higher drug concentration in the cream or lotion
(4) longer application time or frequent application
(5) molecular size of the drug
(6) chemical nature of the drug (lipophilic, hydrophobic, etc)
(7) application of heat
(8) chemical permeation enhancers
The ideal drug has a low molecular weight with high water and lipid solubility.
Factors that decrease skin integrity:
(1) thinning of the skin at the extremes of age (premature neonate, elderly)
(2) thinning of the skin due to drug or hormone effects such as corticosteroids
(3) breakdown in skin integrity, especially the stratum corneum (burns, eczema, other)
(4) malnutrition and hypoalbuminemia
