Disclosed is the use of a pharmaceutical composition comprising methylnaltrexone of formula I, or a salt thereof, in the preparation of a medicament, wherein the medicament is formulated for oral administration once-daily in the morning for treatment of opioid induced constipation in a subject, wherein the pharmaceutical composition comprises 150 mg, 300 mg, or 450 mg of methylnaltrexone, or a salt thereof, and dodecyl sulphate, or a salt thereof； wherein the subject has discontinued alternative laxative therapy.
