Nepal is one of the five Himalayan countries and is surrounded by many mountains that are higher than 8000m. But in fact, most of the historic cities in Nepal like Kathmandu and Pokhara are located at an altitude of 800-1400m. The Chitwan national parks in the south of Nepal are even lower(220m). So no worry about altitude sickness for a trip to the above places.
For Nepal trekking trails of the Himalayas, usually, when reaching 2500m to 3000m some people will feel altitude sickness(headache and trouble breathing), the higher and faster you gain height the bigger the risk becomes. Our local guide had been treated with first aid who will give you the best solution like descending to lower altitude & if the condition is extreme you will be rescued to the local hospital as soon as possible. Travelers with pre-existing heart or lung or other problems should seek for doctor's advice before going on a high altitude(above 3000m) trip in Nepal.