Low blood pressure is a relative term because the blood pressure normally varies greatly with activity, age, medications, and underlying medical conditions.
Low blood pressure can result from conditions of the nervous system, conditions that do not begin in the nervous system, and drugs.
Neurologic conditions that can lead to low blood pressure include changing position from lying to more vertical (postural hypotension), stroke, shock, lightheadedness after urinating or defecating, Parkinson's disease, neuropathy and simply fright.
Nonneurologic conditions that can cause low blood pressure include bleeding, infections, dehydration, heart disease, adrenal insufficiency, pregnancy, prolonged bed rest, poisoning, toxic shock syndrome, and blood transfusion reactions.
Hypotensive drugs include blood pressure drugs, diuretics (water pills), heart medications (especially calcium antagonists-nifedipine/PROCARDIA, beta blockers-propranolol/INDERAL and others), depression medications (such as amitriptyline/ELAVIL), and alcohol.