Hello dear, Thank you for your question. Dizziness is a complex symptom that can have many causes: 1- The most likely cause of dizziness, in patient receiving high BP medicines, is associated with an excessive drop of BP due to an inappropriate dosage of the medicines. Of course that taking BP medicines regularly is very important in managing high BP, but you have to consider that a high dose can lower the pressure to much, and produce symptoms like dizziness, lightheadedness, fainting etc. Despite regular medication, your sister needs to check constantly her blood pressure, and refer to her doctor to adjust the dosage accordingly. 2- Spikes in BP can also manifest with dizziness. Random or a even a more steady elevation of blood pressure due to inadequate medication can present with dizziness, headache, visual disturbance etc. 3- Some BP medication are widely known to cause dizziness as a side effect. Some medications like Beta blockers, Calcium channel blockers etc. are associated with high blood pressure, especially if they are prescribed at high doses. In this case you might need a consult with your doctor to change the regimen to another drug. 4- If your doctor excludes all the above mentioned factors, there are additional factors to be considered: - irregular heart rrythm (arrythmias) - vestibular disorders - heart disease - anxiety disorders. All of the above mentioned factors require a consult with a doctor in order to address the cause of the symptom. Don t forget that dizziness needs special attention in elderly patients, because is associated with serious damage from falls. I hope this helped. Don t hesitate to contact, if you have any further questions. All the best. Dr. Alma