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Lactase is an enzyme produced in the intestine that breaks down milk sugar (lactose). When a person becomes lactase-deficient (also called lactose-intolerant), he or she has difficulty digesting milk and milk products. The undigested lactose is used as food by bacteria that are normally found in the intestine. This can cause diarrhea, a large amount of gas, and cramps.

Giardiasis causes some people to become temporarily lactase-deficient. It may take as long as 1 month to be able to eat milk and milk products again without a problem.

If a person being treated for giardiasis has symptoms that suggest this is a problem, he or she should avoid milk products for at least 1 month. Then add them gradually back into the diet as the body can handle them.