Altitude sickness is very unlikely on the Nar Phu Valley Trek. You will not spend a night above 4000 meters. The highest point you reach is Kangla Pass (5320 meters/17,454 feet), but only a few moments are spent there, so you are not facing a high risk of altitude sickness. That being said, some are more sensitive to the variations in elevation than others. Our team is highly experienced in the case of high-altitude sickness and is properly trained to help you. At the onset of symptoms, our guides will take you to a lower elevation as quickly as possible. You should not worry, though, we have built-in acclimatization days to prevent altitude sickness from happening.