Most of the illegally produced fentanyl represents a highly hazardous drug, much more so then heroin, which itself can be a deadly. So many people die daily, simply because the actual fentanyl concentrations in the final products are unknown and extremely variable. Also, it is usually full of various contaminants and adulterants (including deadly nitazenes), in addition to heroin itself (very often).
These mixtures are made by the people who know nothing of fentanyl potency, and have no means, nor the desire to prepare the proper, and reasonably save products for the end users.
However, pharmaceutical fentanyl is highly efficient and widely used opioid drug, both as injectable solutions and fentanyl patches, and it is used regularly and safely, by millions of patients worldwide.
Those who produce super pure fentanyl (>99.5%, the one far above the actual pharmaceutical grade, which is slightly above 98% pure), and do so outside the legal framework, know exactly how to produce the proper mixtures (with 100% potency of heroin) and have a lot of satisfied (and alive) customers.
Super pure fentanyl (as a citrate salt), a substance well-known to the manufacturing experts (free flowing, snow-white micro-crystals) is then mixed with heroin (titrated to 85-90% purity) in a ratio 1:50, as it is about 50 times more potent then heron. Thus, 100 mg of the mixture, contains 2 (or up to 3) mg of the pure fentanyl citrate and ~100 mg of heroine (85-90% purity), and is about two times more potent than the same amount of heroin alone. It is suitable and reasonable safe for injections, just as the regular heroin (the actual amount used, depends on the customer’s habits and tolerance). In practice, the use of heroine in the mixture is not mandatory, as lidocaine and other solid diluent (about 100 mg per 2-3 mg of the above fentanyl) produce the mixture of the same potency. Generally, snuffing is preferred to injections. Also, the powdered mixture can be placed in mouth, below the tongue, and waiting to be gradually absorb through the mucose. The experience is exactly like a heroin, albeit it last somewhat shorter (it is a drawback, but also a benefit, as it reduces the risk of spontaneous stop of breathing, a common cause of complications and death among heroin users).
Thus, the customers need not to experience any adverse effects, let alone to die from fentanyl. It is all up to the irresponsible and unprofessional manufactures, and even worse dealers. If the whole chain works professionally, including the end users, it is no riskier than heroin, even less. And some chains do work exactly that way, thought not that often, unfortunately.
NOTE: The recreational use of any opioid, heroin or anything else, is not recommended to anyone. If some wants to try it nevertheless, they should start with the very small amounts and by sniffing, not injections! All the opioid users gradually develop dependence which is very difficulty to cure! And the acute risks always exist, particularly the above mentioned stopping of spontaneous breathing (and the resulting complications, including death).