What is the reason for this minhag? The Smak explains that the reason kitniyos are not eaten on Pesach is because of a gezeira (a Rabbinical restriction enacted as a preventative measure.) Since cereals and other similar dishes are made both from kitniyos and the five types of grain, a person, could, in error, come to eat dishes from the five types of grain – which are chametz. In addition, there are places where breads are baked from kitniyos (e.g. some corn breads are from corn, which are kitniyos. Some corn breads are actually made from rye – which is one of the five types of grain.) A person could, in error, assume assume that he is eating bread from kitniyos while it may be from the five types of grain. The Beis Yosef mentions another reason for the minhag, because kernels of legumes and grain may become mixed together on occasion and are difficult to differentiate.
Although Ashkenazim may not eat kitniyos, they are permitted to possess and even use kitniyos during Pesach (unlike actual chametz which is assur b’hana-ah / forbidden to derive any benefit therefrom.) Example: One may use baby powder containing corn starch.
Concerning the question of whether liquids (e.g. corn syrup) and other derivatives of kitniyos (e.g. lecithin which comes from soybeans is used in chocolates and other foods) are included in the gezeira of kitniyos and whether peanuts (and peanut oil) are considered as kitniyos, there are varying opinions among the Poskim/halachic authorities. One should conduct himself according to his minhag. If one is uncertain of his minhag, a Rav should be consulted.
The gezeira of kitniyos does not apply in case of illness. Therefore, medicines containing kitniyos may be used. Most medicine tablets have a base or filler of starch. Therefore, if the starch is from kitniyos (e.g. corn starch) it is permissible in case of illness. If it is from chametz, a Rav should be consulted.”
For more information on kitniyos (including an interesting discussion on quinoa), see http://www.star-k.org/kashrus/kk-passover-kitniyos.htm