Alcohol tolerance - does it diminish or strengthen over time?

I feel as though mine is RAPIDLY falling. I had about 5 pints last night and on the commute this morning I was swaying, close to throwing up, and I'm just about recovering now.

One of my mates in his early 30s does this two or three times a week and it has no visible effects on him or his ability to get into work at about 9 every day.

Tell me about yours. What's going on?