A cocrystal of (2 S ,3a S ,4 R ,5 R ,7a S )- and (2 R,3a S,4 R ,5 R ,7a S )-7-bromo-2-(4-methoxyphenyl)-3a,4,5,7a-tetrahydro-1,3-benzodioxole-4,5-diol (17:3)