Because it's so potent, even a small dosing error can slow or stop breathing; law-enforcement testing shows as little as about 2 milligrams can be lethal depending on a person's tolerance. On top of that, people often encounter fentanyl unknowingly in counterfeit pills or mixed into street drugs, so they can't "dose" it reliably. Importantly, medical toxicologists emphasize that incidental skin contact is extremely unlikely to cause overdose — so it's safe to help someone if you use basic precautions.

Source: US DEA Facts about Fentanyl

Illicitly manufactured fentanyl is frequently blended into heroin, cocaine, and methamphetamine and is also pressed into fake prescription pills that look like oxycodone, Xanax, and others—often without the user's knowledge. This contamination and counterfeiting create unpredictable strength from dose to dose, which sharply raises overdose risk.

Source: National Institute on Drug Abuse

Only take medications dispensed by a pharmacy; avoid pills from friends, dealers, or online sources. Carry naloxone (now available over the counter) and learn overdose response - give naloxone, call 911, and provide rescue breathing.

Source: National Institute on Drug Abuse