Dimers of acetyl salicylic acid, including 4,4′-diacetoxy-[1,1′-biphenyl]-3,3′-dicarboxylic acid (DAS-1) and 5,5′-methylenebis(2-acetoxybenzoic acid) (DAS-2) are provided. Methods of blocking the C3 convertase stage of the alternative complement pathway, preventing formation of the membrane attack complex of complement, and preventing or treating a complement-mediated disorder in a mammal including the step of administering dimers of acetyl salicylic acid are also provided.