Fentanyl is 50 to 100 times stronger than heroin. So it's a perfect choice for suicide I think. The problem is purity. If you buy it from the streets you have no guarantee how clean it is.
The ultimate way for suicide, I think, is using fentanyl patches. But those are very hard to find. Here is an article about a man who killed himself by using fentanyl patches:
https://www.sfgate.com/bayarea/article/Druggist-used-pain-patches-to-end-his-life-2860267.php
Please note that he was a professional druggist, in other words an expert on these things, and he chose to use fentanyl patches for his own suicide. He had access to many different drugs and he, the professional druggist, chose fentanyl patches!
Also...just a thought: On this forum we tend to use codes for certain items, like "N" for nembutal, and now "F" for fentanyl. Why are we doing that? I think we should stop using this codes while on this website. There is no logical or good reason for using them. It will only confuse people. On other websites yes maybe, but not on this site. This is home.