From UW's ADAI:
Drug users and bystanders who witness a drug overdose are often reluctant to call 911 out of fear that they might be arrested for drug possession. Initial results from an evaluation of Washington State's 911 Good Samaritan Law show that 88% of opiate users are now likely to call 911 to get emergency help for drug overdose victims, knowing that they are protected by the state’s Good Samaritan laws.
The study's early findings also indicated that 62% of police surveyed said the law would not change their behavior during a future overdose because they would not have made an arrest for possession anyway, 20% were unsure what they would do, and 14% said they would be less likely to make such an arrest.