Chemically stable pharmaceutical formulations comprising a therapeutically effective amount of a material of formula I are provided which formulations contain a stabilizer system comprising an organic carboxylic acid containing more than 1 carboxylic acid group and an antioxidant. The preferred organic carboxylic acid is citric acid and the preferred antioxidants are ascorbic acid and butylated hydroxytoluene, which antioxidants are preferred to be used in admixture. The chemically stable pharmaceutical formulations are particularly useful for treating Irritable Bowel Syndrome (IBS) in mammals and comprises administering to the mammal needing treatment an effective dose of the above chemically stable pharmaceutical formulation. The stabilizer system is also effective for use in other pharmaceutical formulations containing active ingredients having oxidizable groups such as amino, phenolic, hydryoxyl amino, aldehyde, unsaturated compounds, sulfoxide, sulfone and mercapto. The preferred Formula I compound is as follows: ##STR1## wherein R and R.sup.1 are propyl; R.sub.2 and R.sub.3 are hydrogen; Y is CH.sub.2 and the compound is an HCl salt.