According to a new report from the SF Medical Examiner's office, it's called fluorofentanyl.
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It was found during examinations of 45 people who died of drug overdoses last year.
The strain can be up to five times more powerful than prescribed fentanyl.
The Medical Examiner's office said it did not know how many deaths this year involved fluorofentanyl because it didn't have capacity to conduct that testing.