Why upon standing do i get dizzy been happening for over two months. i have been on atenolol 25mg. once daily for tachycardia

Many reasons: Dizziness after standing up can be due to orthostatic hypotension if your resting BP is too low or you're dehydrated. It can also occur due to bradycardia, or slowing of the heart rate (HR). Atenolol decreases both your HR and BP. You should check your BP and HR twice daily and take the recordings to your PCP for further guidance. http://www.webmd.com/a-to-z-guides/hypotension-orthostatic

?hypotension: You could be having an adverse reaction to your medication, and perhaps orthostatic hypotension. But you could possess anemia, hypothyroidism, dysautonomia, and even a migraine equivalent. Dehydration could cause this, but usually resolves on hydration.