Some handle the lack of oxygen quite well. They blaze up mountains with speed and grace snapping their photos at the peak with ease and fresh smiles.
Others struggle a bit more. They bob and weave and don't even remember where they are. A quite strapping young British man I met on my travels on the mountain struggled to make it to the top. He doesn't remember much of his journey and needed quite a bit of help down. Some, like myself, get extreme headaches. Many vomit.
The one side effect that seems to take hold of most climbers is that of extreme thirst for Kilimanjaro beer. They say if you cannot climb it, drink it. My experience is that if you do climb it, with or without struggle, you end up drinking it, with pride and large quantities. And so the side effects of mountain climbing are as follows:
1. You believe you are a character in Lord of the Rings and sing the theme song in elvish.
2. You tell everyone how much you love them, often.
3. You feel invincible and therefore say and do things that you normally would not, for example an interpretive dance to vanilla ice.
4. You bond over your climb and hug new friends like they are old pals.
5. You return to the days of your youth and find yourself engaging in games called "Ring of Fire".
6. You think you are seriously funny and laugh at everything.
7. You make pledges to meet said new friends in Las Vegas to relive the "glory days".