Athletes and people who are extremely fit tend to have low blood pressure, explains Mayo Clinic. Conditions that can cause low blood pressure include heart issues, such as heart valve problems and heart attack; endocrine issues, such as thyroid disorders, Addison's disease and low blood sugar; and significant blood loss. Dehydration and diets lacking in vitamin B-12 and folate can also cause low blood pressure.
Many women experience a drop in blood pressure during pregnancy, with blood pressure levels typically returning to normal following child birth, reports Mayo Clinic. Septicemia, which occurs when an infection enters the bloodstream, and anaphylaxis, which is a severe allergic reaction, can both cause drops in blood pressure.
Medications that can cause low blood pressure include diuretics, alpha blockers, beta blockers and drugs prescribed to treat Parkinson's disease, according to Mayo Clinic. Tricyclic antidepressants, such as trimipramine, protriptyline and doxepin, also sometimes reduce blood pressure.
Systolic blood pressure is the first number in a blood pressure reading, and diastolic blood pressure is the second number in a blood pressure reading. As of 2015, normal blood pressure is under 120/80 millimeters of mercury, or mm Hg. Blood pressure levels under 90 mm Hg systolic or 60 mm Hg diastolic are considered below normal, states Mayo Clinic.