The use of two or more drugs in the same prescription is a common practice for the treatment of a patient, which may sometimes be neither safe nor effective. The main aim of the present study was to find out invitro and invivo drug- drug interaction study between ketotifen fumerate and chlorpheniramine maleate in aqueous media at various pH (1.2, 2.8 and 6.8) by using Job‟s continuous-variation analysis and Ardon‟s spectrophotomeric methods. The stability constant values of chlorpheniramine with ketotifen were determined at a fixed temperature (37 0C) at both gastric and intestinal pH. The stability constant values , ranging between 7.79 and 8.21 when ketotifen fumerate and chlorpheniramine maleate were the drug of choice, were derived from Ardon‟s plot as a result of interaction between the drugs, were comparatively stable. However, after interaction it was observed that, stability constant values were more than 1 at both gastric pH (1.2 and 2.8) and intestinal pH (6.8). The in vivo study indicate that the decrease in concentration of ketotifen after concurrent administration of ketotifen and chlorpheniramine was statistically insignificant.