Furanocoumarin derivatives (dimers and monomers) present in commercially available grapefruit juice have the capacity to inhibit the activity of human CYP3A4. Such interactions are believed to result from the mechanism-based inhibition of CYP3A4 activity in the intestine. The aim of this work was to synthesize and test a series of dimers with a view to determining the relationship between structure and inhibitory activity and determining whether they might make suitable probes of CYP3A4 activity. We prepared a series of furanocoumarin, coumarin, and benzofuran derivatives that have inhibitory effects on the activity of human CYP3A4. A synthetic benzofuran dimer, which is more accessible than furanocoumarin dimers, exhibited activity against CYP3A4 comparable to that of furanocoumarin dimers.