What Causes Dizzy Spells?

Hi doc. My aunt is having a problem like her head is spinning sometimes. what could be the reason for that ? she had consulted a doctor and he she had done mri and there was no problem in that. He gave some medicine and said she has to continue the medicine as he said that the blood flow to the brain is less. But when she takes the medicine she feels weak and moody and drowsy . Could you give your comments please .