In May 2017 the Good Samaritan Act for Drug Overdose, was passed in Canada.  It provides some legal protection for both the drug overdose victim and the person phoning 911 from:

1. Simple possession
2. Parole violations
3. Pre-trial conditions
4. Probation orders
5. Conditional sentences
 
The act does not provide legal protection against more serious offences, such as:

1. Outstanding warrants
2. Production and trafficking of controlled substances
3. All other crimes not outlined within the act
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The above applies to the person who calls 911 and the overdose victim.
 
 See Health Canada's webpage describing the Act.