(AP) — Eighteen people in Milwaukee have died of overdoses this year involving a fentanyl derivative that's so potent it can require higher doses of an opioid antidote to counteract it, the county medical examiner's office has said.

The deaths come as the Drug Enforcement Administration prepares to label acryl fentanyl as a Schedule I drug as soon as Monday, which would classify it as having a high potential for abuse. The overdoses also highlight a troubling increase in the number of deaths related to fentanyl or synthetic fentanyl in Milwaukee.

"It's been trending exponentially," said Sara Schreiber, the forensic technical director of the medical examiner's toxicology lab. "It's been very scary."

This year, 59 overdoses have involved fentanyl, including those from acryl fentanyl, putting the county on pace to exceed the 97 deaths related to the drug last year, according to Schreiber. The drug killed 30 people in 2015, she said.

In 2012, only five deaths were linked to fentanyl.

Schreiber said deaths from acryl fentanyl are due to the drug becoming more available and increasing in potency.

Acryl fentanyl is 100 times more potent than morphine, requiring multiple doses of the opioid antidote naloxone, known by the brand name Narcan. Schreiber said the challenge for paramedics is that they sometimes don't know they're dealing with a synthetic opioid that has such a strong potency.

"Enough (Narcan) has to be given in order to counteract that, and you have to get there quickly enough," she said.

Schreiber said Milwaukee has also seen a handful of deaths this year involving carfentanil, which is used as a tranquillizer for large animals, such as elephants.

Acryl fentanyl is not as strong as carfentanil, but Robert Bell, the DEA's assistant special agent in charge in Milwaukee, said all fentanyl derivatives should be considered deadly, even in small doses.

"They're all bad, they're all dangerous," Bell said.

Manufacturers of synthetic drugs can modify their formulas to skirt state and federal law prohibitions, but the DEA's move to label drugs such as acryl fentanyl as Schedule I classifies them as drugs with no accepted medical use and the highest potential for abuse.

The DEA filed its notice of intent to temporarily label acryl fentanyl as a Schedule I drug on June 2, and it takes 30 days for it to be officially listed. The listing is temporary while the agency evaluates the drug further, but Bell said the intent is to make it permanent.

A bill pending in Wisconsin seeks to make all types of fentanyl a controlled substance, including future iterations, unless they have medical value. The state's Controlled Substances Board already listed acryl fentanyl as Schedule I drug in May.

Milwaukee has had 147 total drug overdoses this year, which include fentanyl deaths. Last year, the total number of drug overdoses was 343 and it was 255 in 2015.