Lethality of Opioids
Here is an excellent article from Scientific American explaining in very simple terms how opioids exert their lethal effects. It also explains why the more potent opioids such as fentanyl pose an even greater threat to life. The usual doses of naloxone that are used to try and reverse an overdose might not be sufficient when heroin has been mixed with fentanyl.
How Opioids Kill