age, I know exactly how I would feel if this came up between my parents and me. The lack of trust and betrayal I would feel would be absolutely astounding.
I do have friends like Alison (I'm guessing). Thankfully, none of us have died. I do not, however, believe drug use is unconditionally bad. I think that it is all part of life. The important part is being (relatively) responsible about it. If Alison is anything like my friends, then they might not be the brightest or have the most common sense. They might also make some poor choices.
I think that there are better ways to confront this issue than by spying on her. For instance, TALKING to her might be a good place to start. It might not seem like it works, but as long as you aren't (what we perceive to be) an asshole about it, more of it gets through than you might think.
Instead of saying "No drugs or alcohol" (which will make you sound like the aforementioned asshole), be reasonable. Be an informed person, and be willing to DISCUSS the potential dangers and benefits of certain drugs (marijuana, for example, can have amazing benefits with very few repercussions as long as you keep the law out of it). Tell them, "I would prefer it if you didn't do 'y', but sometimes things happen." Make sure that they know about what they might take. Help them understand differentiating substances and the respective drug's appropriate dosage (of course, these are always approximations as everyones body is different, so error on the lower side). Tell them not to drive under the influence, and that, if the need arises, you would be more than willing to give them a ride (as long as they clean up the throw up that might result in the backseat). Let them know that if they have a problem, or think that something might be becoming a problem, that you will be there to offer your support for them, because after all, you do love them even if they did do something stupid.
All of us are looking for new experiences. Trying to lay down a blanket law that prohibits them and offers serious repercussions if these rules are broken are not going to do much for many teens, and certainly not those like Alison. I understand that at heart, parents do want the best for their children, but I think that sometimes the means used aren't the most helpful or the most effective.
You can't always shield us from pain. Sometimes people have to experience pain in order to live and beyond even that, to enjoy the life that they have. Part of being a parent (from most of my friends perspectives) is being their to help cushion the blows that may come, not stand in front of the swing to block it only to turn around and slap us for the punch being thrown in the first place.
I sometimes consider it similar to abstinence only education. Its hard to talk to your children about sex and what they should know about it, and of course it's much easier to tell them to just wait, but do you really think that that will happen for the majority of kids?
It all comes down to trust and rationality. Be a rational person, and be willing to trust your children. In turn, respect and accept the trust they will place in you. It will take time and possibly an attitude change, but in the end I think you will both be happier people. She will have her freedom, and you will be able to know that she is informed and knows what her actions can do.
So, after this long post, a sentence or two summary- Spying on her will make your life more difficult as you try to keep tabs on her 24/7, and she will also refuse to trust you even more, and if she is indeed like my friends, she will continue her actions, even if it is bad for her, just to piss you off and prove to you that she can.
ON EDIT: Should you want to inform yourself a little more (or perhaps take a look back and reminisce if you are already well versed in drug use),
http://www.erowid.org is a great place to learn more about drugs, their history, and their usage.