Babies who have been on formula since birth and are four months old may also develop lactose intolerance, which is mostly considered to be secondary. Lactose intolerance refers to the inability to fully digest the lactose in dairy products and symptoms such as bloating and diarrhea may occur after eating or drinking dairy products. Lactose intolerance can occur at four months of age and is usually considered secondary. Secondary lactose intolerance is caused by damage to the small intestine due to various reasons. And common causes of this problem include gastroenteritis, diarrhea, intestinal parasites and other environmental factors. In congenital lactose intolerance, babies are born unable to make lactase due to the absence of a recessive allele on the autosomes. It is recommended that babies be fed properly after birth, and if lactose intolerance develops after a few months, prompt medical treatment is needed.