Altitude Sickness: Cuzco is approximately 3,400 meters (more than 11,000 feet) above sea level, and travelers need to inform themselves about altitude and acute mountain sickness (AMS) or "soroche", which may be prevented and treated under the advice of a doctor BEFORE travel begins. Although the Sacred Valley is nearly 2000 feet lower than Cuzco, this is still high enough to affect people who are susceptible to altitude sickness. Everyone does not react the same way to higher elevations. However, the best way to abate the effects of altitude sickness is to go to a lower altitude, and this is why many of our suggested  itineraries begin by transferring immediately to the Sacred Valley  after landing at Cuzco Airport, and then returning to Cuzco later in the trip, once you've acclimated to the altitude.
Prescription medications are available to prevent or reduce the affects of Altitude sickness, but they need to be taken before arriving at higher altitudes, so please contact your health care professional prior to leaving.