Lactose is a reducing sugar consisting of galactose and glucose, linked by a β 
(1→4) glycosidic bond, considered as an antioxidant due to its α-hydroxycarbonyl 
group. Lactose is widely ingested through the milk and other unfermented dairy 
products and is considered to be one of the primary foods. On the other hand, 
lactose is also considered as one of the most widely used excipients for the 
development of pharmaceutical formulations. In this sense, lactose has been 
related to numerous drug-excipient or drug-food pharmacokinetic interactions. 
Intolerance, maldigestion and malabsorption of carbohydrates are common 
disorders in clinical practice, with lactose-intolerance being the most 
frequently diagnosed, afflicting 10% of the world's population. Four clinical 
subtypes of lactose intolerance may be distinguished, namely lactase deficiency 
in premature infants, congenital lactase deficiency, adult-type hypolactasia and 
secondary lactase intolerance. An overview of the main uses of lactose in human 
nutrition and in the pharmaceutical industry and the problems derived from this 
circumstance are described in this review.