- Fentanyl is only more dangerous than heroin because it is not properly prepared. This means the dilution is not homogeneous, and users unexpectedly come across a stronger dose or batch.
- If properly prepared, fentanyl is safer than heroin. Its therapeutic index is 400 vs 70 for heroin (this measures the ratio between active dose and toxic dose). Proper preparation means diluting it so that users can take a dose similar to heroin for the same effects. With correctly diluted fentanyl, users have a wider margin of error before an overdose. Dilution is never done as a last step by the user, it's done in labs that prepare large amounts of the drug.
- For addicts, the ritual is the same for both drugs. The feeling is mostly the same, but lasts a little shorter for fentanyl. The addiction potential being higher for one or the other is disputable.
- Once again, it having a lower active dose ("stronger") does not make it inherently more dangerous. It just makes dilution more technical, and this is where the problem comes from.
Edit: Of course, I am in no way defending recreational use of the drug. Like all opioids, it's very addictive and can easily lead to a self-destructive habit. And with recreational users, overdoses will probably happen even with a perfectly diluted batch.