Do you suspect that you will have a problem? Last January I got hit hard with altitude sickness. On the way to Vail for skiing I had no issues in Denver. On the drive up, passing through 9,000 it hit me immediately. Wicked headache and general malaise, just devolved from there to the point of vomiting. Never really got to ski. Should have gone to the clinic (dumb on my part). A few days later back in Denver, 90% resolved in two hours. If you have a strong reason to anticipate problems, speak to your doctor about taking "standby" medication with you (google it).