Provided is the use, in a pharmaceutical composition, of an absorption enhancer comprising polyethylene glycol 660 hydroxy fatty acid ester as an agent for enhancing absorption of a therapeutic agent across a mucosal membrane, wherein the therapeutic agent has a log P value less than about 3 and: (a) is a peptide, protein, nucleic acid, antigen or vaccine or (b) is not a substrate for P-glycoprotein (P-Gp).