The photorefractive effect in metal-insulator-metal (MIM) plasmonic waveguides is modeled and investigated for both symmetric and asymmetric structures. A metal--metal nanostructure with different top and bottom metals is proposed as a plasmonic waveguide. Three different metals, aluminum, gold, and silver, are used as the metals of the plasmonic waveguide and the photorefractive effect is investigated in different structures. The interaction between interfering surface plasmonic polariton modes is studied. It is shown that a strong mode can be coupled to a weak mode by means of the photorefractive effect. It is found that compared with conventional symmetric MIM plasmonic waveguides, an asymmetric structure with different metals, silver--gold for example, has a higher photorefractive gain and longer propagation distance, and there is a net photorefractive gain. The effect of the crystal thickness and amplitude inputs is also analyzed. It is found that there is an optimum crystal thickness that yields the maximum photorefractive gain.