The compound 1-[2-(2,4-dimethylphenylsulfanyl)phenyl]piperazine (compound I) has been disclosed in WO 03/029232 as the free base. The present disclosure is directed to the finding that the L-lactic acid addition salt (=L-lactate), the D-lactic acid addition salt (=D-lactate) and the DL-lactic acid addition salt (=DL-lactate) of 1-[2-(2,4dimethylphenylsulfanyl)phenyl]piperazine are highly soluble whereas the usefulness of other known forms is compromised by poor solubility.