There are two ways to understand this term. On one hand, it is a condition characterized by the inability to absorb or assimilate certain nutrients or drugs, so that exposure to the untolerated substance may cause an adverse reaction. For example, it is common for transplanted hematological patients to develop intolerance to lactose due to the drug treatment they receive. On the other hand, there is another form of intolerance that is very common in oncohematological patients, which is intolerance to activity. It is a lack of physiological or psychological energy to perform or complete normal daily activities, as there is feeling of tiredness or weakness.