OK....you're brand new...
You are asking a very general question about a serious medical situation.
If you are suspecting yourself, a family member or friend of drug abuse..you need to seek guidance from a responsible person who can help beyond that of a public internet forum.
If it is your family member...you can contact you parents (if you are a child - minor or otherwise), other trusting family members, AlAnon, NarcAnon, etc...
Otherwise...you need to have a frank discussion with the individual who you are concerned about or someone who has significant influence with him/her...parent, medical provider, conselor....but not us here.
Best of luck!