This one photo depicts why fentanyl is so much deadlier than heroin

New Hampshire State Police Forensic Laboratory has released an image that contrasts the lethal doses of the synthetic opiate fentanyl and heroin. They even went as far as to compare using fentanyl to "injecting yourself with a loaded gun".

Fentanyl is prescribed for severe pain, such as from cancer. However, dealers don't find it hard to get their hands on this deadly drug and often resort to mixing it with their heroin to get more bang for their buck. Heroin is mixed and distributed with this omnipotent chemical, resulting in a far stronger, and far more dangerous product. Therefore, the unaware user poses a significantly greater risk of overdosing.

Source: https://www.statnews.com/2016/09/29/fentanyl-heroin-photo-fatal-doses/