Hi. when i stand up suddenly, my eyes black out and i get dizzy for a few seconds. why does that happen?

See below: There are many caused of transient drop in blood pressure on standing, but mostly, it is a normal reaction. Make sure that you are properly hydrated. Drink enough water every day so that your urine is mostly colorless. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Drink enough water daily, so that your urine is mostly colorless. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex, if you have sex. Get HPV vaccine

Typically, blood pressure drops transiently when standing, but usually no syncopal symptoms. You may be dehydrated, have orthostatic hypotension, have anemia or thyroid issues, but if taking medication, this could be answer. Have your doctor check this out and obtain supine and erect blood pressure readings.