Chemotherapy or Radiotherapy
Often required for the treatment of cancer, chemotherapy and cranial radiotherapy reduce or stop cell activity in the lumpy follicles and sebaceous glands.  The hair falls a few days after the start of treatment. The scalp can become sensitive, irritated and dehydrated during treatments.  When treatment is complete, the hair repels.  The capillary cycle is restarting, but the hair still weakened is therefore more likely to be damaged by treatments such as colouring, and certain hair products.
The importance of hair loss and how fast they grow back varies from person to person.  Every treatment and every patient is different.