I have recently finished a Commerce degree, and have been using drugs for the last couple of years.
My drug use is fairly infrequent, and I don't think it had a negative impact on my studies at all. In fact the last couple of years I would say have been the most productive in all facets of my life.
I think it is all about responsibility. In that sense, illicit drugs probably shouldn't be treated much differently to alcohol. If you use them too frequently, they will screw up your grades at uni. If you have an exam or an essay the next week, you probably shouldn't be writing yourself off on the weekend. If you are responsible enough to not do this, then you probably can take drugs during uni without messing with your grades. If you are the type of person who will put the university responsibilities second, because the opportunity for a huge night has come up, then you should probably be saying no drugs during semester.
You are your own judge of whether or not you can handle taking drugs during semester. I'm sure you know how long it takes you to bounce back after taking ecstasy and feel completely functional etc.
Sorry about the possible incoherence of this post. I should be asleep.
bRAVEheart