Fentanyl has transformed drug busts and the overdose crisis. Seizures that used to be measured in kilos of heroin are now measured in pounds of fentanyl or millions of pills, because a tiny amount goes a long way. To understand why, you have to understand potency.

The potency ladder

These comparisons are based on DEA and CDC descriptions. They are approximate, since potency comparisons depend on how they’re measured.

Why 2 milligrams matters

DEA considers 2 milligrams of fentanyl to be a potentially lethal dose. That’s roughly the amount that would fit on the tip of a pencil. The exact amount that kills someone varies with body size, tolerance, and other drugs in their system, but the 2 mg benchmark explains why fentanyl is so dangerous: it takes very little to cause an overdose, and illicit drugs are mixed unevenly.

It’s also why agencies sometimes describe busts in “lethal doses.” A kilogram of pure fentanyl holds about 500,000 doses of 2 mg on paper. See our guide on how agencies calculate street value and doses for what those figures do and don’t mean.

Fentanyl

Pharmaceutical fentanyl is a legitimate Schedule II medicine used for severe pain and anesthesia. The fentanyl in drug busts is almost always illicitly manufactured, typically made with precursor chemicals and smuggled into the U.S. as powder or pressed into counterfeit pills. It’s often mixed into heroin, cocaine, and methamphetamine, sometimes without the buyer knowing.

Because it is so potent, fentanyl is easier to smuggle than heroin: a small package can be worth as much as a large load of another drug. That’s one reason federal law sets a 5-year mandatory minimum at just 40 grams of fentanyl, compared with 100 grams of heroin (see federal penalties by drug and weight).

Heroin

Heroin is made from morphine, which comes from the opium poppy. It is Schedule I, with no accepted medical use in the United States. In much of the country, fentanyl has largely replaced heroin in the illicit opioid supply, and many people who think they’re using heroin are actually using fentanyl or a mixture.

Carfentanil

Carfentanil is a veterinary drug meant for sedating very large animals. It is not approved for use in humans. Even a microscopic amount can be deadly. It has appeared in illicit drug supplies in waves, and its presence in a seizure is a major public safety alert.

Fentanyl analogues

Chemists can tweak fentanyl’s structure to create analogues like acetylfentanyl or furanylfentanyl, some weaker and some stronger. Federal law sets lower trafficking thresholds for fentanyl analogues, and DEA has placed fentanyl-related substances as a class under scheduling controls to keep up with new variants.

Can you overdose by touching fentanyl?

This is a common question after officers report “exposures.” According to medical toxicology experts, the risk of a serious overdose from briefly touching fentanyl on intact skin is extremely low. Fentanyl is not readily absorbed through the skin in a way that causes rapid overdose. Agencies still use gloves and precautions, especially to avoid inhaling airborne powder or getting it into the mouth or eyes.

Naloxone

Naloxone (Narcan) can reverse opioid overdoses, including fentanyl overdoses, by blocking opioid receptors. Because fentanyl and carfentanil are so potent, more than one dose may be needed. Naloxone does not reverse non-opioid drugs like xylazine, but experts still recommend giving it in any suspected overdose, since opioids are usually involved too. It is available without a prescription.

The bottom line

Fentanyl is roughly 50 times stronger than heroin, carfentanil roughly 100 times stronger than fentanyl, and as little as 2 mg of fentanyl can be deadly. Those numbers explain why fentanyl seizures are reported in pills and milligrams, why penalties kick in at such small weights, and why the drug has dominated U.S. overdose deaths.

Frequently asked questions

Is fentanyl stronger than heroin?

Yes. Fentanyl is roughly 50 times more potent than heroin and about 100 times more potent than morphine, according to DEA.

How much fentanyl is a lethal dose?

DEA considers 2 milligrams a potentially lethal dose, though the actual amount varies with tolerance, body size, and other drugs taken.

Does Narcan work on carfentanil?

Naloxone can reverse opioid overdoses, including carfentanil, but because carfentanil is so potent, multiple doses may be needed. Always call 911.

If you or someone you know is struggling with substance use, the SAMHSA National Helpline is free, confidential, and available 24/7 at 1-800-662-4357.