Stringency
The Torah specifies the punishment of karet (spiritual excision) for eating chametz, one of the highest levels of punishment in Jewish tradition. During Passover eating chametz is prohibited no matter how small a proportion it is in a mixture, although the usual rule is that if less than 1/60 of a mixture is not kosher the mixture is permitted. If the dilution happened before Pesach then the usual 1/60 rule applies; however Ashkenazi Jews apply this leniency only if the mixture is liquid.
Also, hana'ah (any benefit, such as selling) from some forms of non-kosher food is permitted, but no form of benefit may be derived from chametz during Passover. Mixtures containing less than 50% chametz, and which are not eaten by normal people (e.g. medicine or pet food, even if it is perfectly edible), may be owned and used on Passover, but may not be eaten.
Read more about this topic: Chametz