system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If just a couple "specks" of fentanyl is enough to kill a person, and one can overdose merely by touching it, how is it that people are able to ingest it without dying?

I'm hearing more and more about police officers and other people who have came into contact with it and have either died, or nearly died of an overdose, just by touching it. So how are people even able to ingest it without immediately overdosing? I remember an episode of *Intervention* and this girl was addicted to prescription fentanyl "lollipops". Obviously, the answer is that there must be an extremely small dose in them, but if you can die from a "speck", what kind of dose are we talking? 1/100th of that?
gpt
As always, the dose makes the poison, and your mileage may vary. Fentanyl patches are meant for patients receiving large chronic doses, as in cancer pain, where tolerance is already high. If a child, weighing a third of what the intended patient does and never having received opioids, sticks the patch on, there's risk. But plenty of people would take that same dose without dying. Lethal doses are usually given as the amount required to kill 50% of people, because it's just not predictable for any individual.

In a controlled medical setting, giving a one tenth of a milligram of fentanyl isn't much different than giving ten milligrams of morphine. For that matter, we use botulinum toxin (Botox) for all kinds of medical purposes, and it's one of the deadliest substances by weight in the world! As long as you can be sure the dose is accurately tiny, it's mostly an arbitrary number. On the flip side, taking 500 milligrams of Tylenol is very safe by any measure. Yet precisely because people aren't afraid of Tylenol, and because it's so widespread, it's a leading cause of liver damage. 

The other thing is that opioids usually kill by shutting off your drive to breathe. In an intensive care unit, where most patients are on ventilators, they don't get that option. High doses of fentanyl are used safely every day in the ICU for pain control and sedation.