Tramadol hydro-chloride-benzoic acid (1/1).

In the cation of the title co-crystal salt {systematic name: [2-hydroxy-2-(3-meth-oxy-phen-yl)cyclo-hexyl-meth-yl]dimethyl-aza-nium chloride-benzoic acid (1/1)}, C(16)H(31)NO(2) (+)·Cl(-)·C(7)H(6)O(2), the N atom is protonated and the six-membered cyclo-hexane ring adopts a slightly distorted chair conformation. The dihedral angle between the mean planes of the benzene rings in the cation and the benzoic acid mol-ecule is 75.5 (9)°. The crystal packing is stabilized by weak inter-molecular O-H⋯Cl, N-H⋯Cl and C-H⋯π inter-actions, forming a two-dimensional chain network along the b axis. The benzoic acid mol-ecule is not involved in the usual head-to-tail dimer bonding, but instead is linked to the ammonium cation through mutual hydrogen-bonding inter-actions with the chloride anion.