A transdermal drug delivery system comprising: (a) a therapeutically effective amount of a non-steroidal anti-inflammatory drug; (b) at least one dermal penetration enhancer, which is a safe skin-tolerant ester sunscreen of formula (I): wherein R1 is hydrogen, lower alkyl, lower alkoxy, halide, hydroxy or NR3 R4; R2 is a C8 to C18 alkyl; R3 and R4 are each independently hydrogen, lower alkyl or R3