A bad trip

A bad trip is a very disturbing and dangerous experience.
Your friend might be feeling bizarre and frightening hallucinations, loss of control, distortions of body image, among others.
Nonetheless you can help him. What you can do is:

• Remain calm and firm
• Change your location, take him/her outside to a calm place
• Change the music to something light and familiar
• Talk to him/her and keep them company
• Don't leave him/her alone but don't crowd them either
• Don't give him/her any medicine  

An overdose

Witnessing an overdose can be a very frightening experience, but you may be the only one that can save that person's life.

• Call for an ambulance. Getting help fast is the first important step
• If the person is unable to breathe, help them out. Check the following link
• If they are breathing, make them lie on their side. If they throw up, this can prevent them from choking or inhale vomit into their lungs
• Stay close to the person and keep them warm
• Don’t let the person take a cold shower, ice bath, etc... It may put them into shock
• Don't force them to vomit, it may cause them to choke or inhale vomit into his/her lungs