if you are in a group of friends and any of you are taken badly ill after taking drugs (whether you have used them before or not) – please call the Emergency Ambulance first (999, 112, 911 or your local emergency number) rather than searching online here or anywhere else!
Be honest with the medical staff (they are not cops, and even if cops do turn up with the Ambulance they are now trained to keep their distance if there is no safety risk to the paramedics) and keep hold of any leftover substances to hand over to them so they can get them tested.
If your friend must go to hospital make sure someone can also visit them and (I know this can be awkward) it is better to notify their siblings or parents yourself than have the hospital staff or even cops do that.
Many overdoses/adverse reactions if caught in time are survivable, provided the patient gets to hospital with the correct facilities and trained staff. But it is not possible to give out medical treatment via the Internet!