As far as I know the whole benzimidazole group is poorely water soluble but sensitive to acid pH of the stomach where disintegrates to smaller suspension particles and further goes to the intestine for absorption. The absorbed moeity ungergoes the intensive first pass metabolism in liver and quickly converts into metabolites. Therefore it coulb ne called stable in the stomach until absorbed and metabolised in liver.

Dear Rosic,

thank you for the reply and giving me your precious time 

Can you provide me the some research articles stated instability into gastric pH

Mebendazole belongs to N-carbamate functional Me group. Under acidic condition, carbamates are not stable and tends to degrade to the amine function, losing the carbamate group.