Slightly tangential - the safety of fentanyl isn't significantly different from any other opiate. The issue isn't the chemical itself but rather the huge differ

Slightly tangential - the safety of fentanyl isn't significantly different from any other opiate. The issue isn't the chemical itself but rather the huge difference in dosage combined with the unreliable nature of the black market. If you know what you have is a specific concentration of fentanyl, you can measure out a safe dose. If you know what you have is a specific concentration of heroin, you can do the same. If what you have could be either fentanyl or heroin and you have no idea what the concentration of either might be, then there's no straightforward way to consume it safely.