Naloxone (or “Narcan®”) is the antidote that helps reverse opioid overdoses. You can administer it intramuscularly, by injection, or intranasally. There is no risk to administer naloxone to anybody, even if they're not overdosing or used other types of substances.
You can get it for free in Quebec pharmacies and in certain community organizations. Visit this page to find out which ones! It is possible to ask the person who gives you the kit to explain how to use it or to follow a training course given by an organization in your neighborhood. If you need to refresh your memory, the steps are well detailed here : Rescuing a person from a possible opioid overdose