Hi, In a new study done from the heights of Mount Everest, Italian researchers found that your blood pressure steadily increases if you ascend to great heights and also found that a drug widely used to treat high blood pressure was ineffective once climbers reached above a certain height. The blood pressure lowering drug called Telmisartan that belongs to the group of antihypertensive medications known as angiotensin receptor blockers or ARBs was used among the climbers (Study Finds High Altitude Affects Blood Pressure – WebMD). Again, blood pressure may be increased due to oxygen deprivation that leads to the heart working harder and blood pressure may not be able to control by the prescribing antihypertensive medication. According to some clinical studies, Losartan HCTZ has no effect on high altitude or lost effectiveness in high altitude but maybe needed higher dose to control high blood pressure due to oxygen deprivation and higher overload of the heart. Again, antidepressants like Venlafaxin HCL ER may loss its effectiveness in high altitude according to some clinical studies but there are lack of evidence based clinical studies. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Zubayer Alam, General And Family Physician