Feeling weak can be caused by a wide variety of factors and conditions. Common causes of general weakness include: medical conditions, such as the flu, thyroid disease, anaemia, diabetes, heart failure, vitamin B-12 deficiency, depression or anxiety a lack of sleep medication side effects certain muscle diseases or loss of muscle e.g. from inactivity chemotherapy reduced physical fitness Weakness in specific parts of the body may be caused by something different. Common causes of weakness in specific muscles include: stroke nerve damage multiple sclerosis spinal disc rupture pressure on spinal cord