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In general, when a forbidden food becomes mixed with a permitted food, it is batel be-shishim that is, rendered insignificant if the forbidden food is less than one sixtieth of the mixture; in such a small quantity, it cannot be tasted. At the level of Torah law, ĥametz on Passover is also batel be-shishim. However, the Sages ordained that even a drop of ĥametz renders food forbidden. This applies even if there is a thousand times more permitted food than ĥametz. Therefore, we are extremely careful about anything that might be ĥametz. Not only do we avoid buying products that lack kosher for Passover certification, but a number of stringencies have become common, some recent and some dating back centuries.
Egg matza is made out of flour kneaded with fruit juice rather than water. (In rabbinic Hebrew, it is called matza ashira, literally “rich matza.”) Strictly speaking, it may be eaten on Passover, as halakhically only water can bring about fermentation. However, due to a variety of concerns, many communities have a custom not to eat it. An additional stringency some groups follow is to avoid eating matza (or matza meal) soaked in water.
Medicines which taste good, such as some syrups and chewable tablets, require kosher for Passover certification. However, in cases of necessity, they may be taken on Passover even if not certified. Bitter or tasteless medicines do not require certification. Soaps and creams do not require certification since they are inedible. Some people are meticulous, making sure to use creams and soaps without a drop of ĥametz in them. Toothpaste must be kosher for Passover, as it is flavored. Dishwashing soap does not require certification since it tastes bad.
