Fentanyl kills for the same reasons that opiates kill – they slow respiration until it stops. Death is by suffocation. Drugs administered by prescription or in hospital environments are quality controlled and used where the effects can be monitored and action taken if something goes wrong. By definition there is no such control in the illegal drugs trade. What is sold on the street could be anything from a little heroin, diluted with something fairly innocuous, to pure fentanyl.