Hi Steph, I'm sorry you're going through this. I found it very upsetting and worrying when my DS1 was taking 'club' drugs. He was a bit younger: 12 or 13 when he started smoking cannabis, 14 when he tried other stuff, and 16 at the 'peak' of his drug-taking.
Like Clay, I used Frank to get information. The newer chemical drugs like ketamine and m-cat etc. are the most dangerous, because they have no real idea what's in them, and no-one knows the long-term effects. The risks of mdma, speed and cocaine depend on quality and contamination. Cannabis is least dangerous, but skunk is nasty stuff. Frank will give you better details.
You will almost certainly have a young person's drug service in your nearest town, and they will talk to concerned parents and give info without asking for names/personal details. I found it very useful to get calm, neutral advice, and it helped me to gain perspective and to 'balance' the disapproval and panic I heard from other people.
Also, more controversially, I made a point of talking to a couple of adults whom I knew had taken similar drugs. I found it hugely reassuring to see for myself that they were living perfectly respectable 'grown-up' lives, because my emotions were screaming "DS's life is totally ruined! He's going to end up in the gutter, or die"... but talking to these people let me get my fears under control a bit.
Also, one of these people offered to talk to my son, which meant that he had a well-informed adult with more 'street cred' than me if he wanted/needed one. When she told him he was doing something dangerous or being a dick, her words seemed to carry more weight than mine.
Of course, not everyone has acquaintances who can do this, but if you do, I'd definitely say it's worth talking to him/her/them.
Personally, I felt pretty powerless, but I do think it was useful to be able to direct my DS towards accurate info, and also to stay calm (enough) and keep communication open. I feel open, honest communication is really valuable - potentially life-saving. We had a couple of incidents (two drug-related, two alcohol) where, because he felt able to be honest with me, he told me details about what he'd taken and how much, which enabled me to assess risk and, once, get medical help.
Just to offer some hope: my DS has now stopped taking 'club' drugs, I believe. For a while, he was messing up his life pretty badly (not going to school/college; failing exams; getting arrested; getting involved in minor crime - all drug-related IMO) but now he is back on track, doing well at college, applying to uni and working in a p/t job. Thank goodness. He seems to have grown out of his risky (or riskiest) behaviour. I hope your DD does too.