A sustained-release, oral pharmaceutical formulation of tramadol comprising a compound formed in situ of tramadol or its salt and a pharmaceutically acceptable acidic substance. The compound formed in situ has a desired water solubility. Also provided are methods of treatment using the pharmaceutical formulations. Method for preparing such formulations are also provided. The preparation method comprises repeatedly mixing tramadol or its salt with the acidic substance, and moistening the mixture and formulating the mixture under an energy input, such as heat or pressure. Optionally, drying, repeated granulating, extrudation and pelleting may also be included.