The main cause of altitude sickness is going too high too quickly. Given enough time, your body will adapt to the decrease in oxygen. This process is known as acclimatization and generally takes one to three days at any given altitude., Cusco is at an elevation of 3,400 meters / 11,154 feet, so spending at least 2 days before any trek will help you a lot. You may also consider taking altitude pills. We often recommend to our clients DIAMOX or Acetazolamide, which are used to prevent and reduce the symptoms of altitude sickness. This medication can decrease headaches, tiredness, nausea, dizziness, and shortness of breath that can occur when you climb quickly to high altitudes (generally above 10,000 feet/3,048 meters). It is particularly useful in situations when you cannot make a slow ascent. The best ways to prevent altitude sickness are climbing slowly, stopping for 24 hours during the climb to allow the body to adjust to the new height, and taking it easy the first 1 to 2 days. (Regarding any specific medications, Please, check about side effects, precautions, interactions, or overdose or ask for advice from your doctor).