I purchased altitude tablets over the counter in Lima and was fine. Was very short of breath in Cusco and some sights around Cusco though, but it was manageable. My father didn't take any medication and he was fine - just had to take it easy. I did Machu Picchu Mountain (vs Huayna Picchu) and managed to really race up - I was in the first half dozen or so that got to the top. I am really fit though... I felt the altitude was very easy to manage.