US authorities seized enough fentanyl to kill more than 42 million people, according to the United States Drug Enforcement Administration (DEA).
Border Patrol agents have arrested two US citizens in connection with the bust, which uncovered 340 packages of pills containing the potent opioid.
A statement shared by the federal agency reports that Tucson Sector agents found the stash at around 6:00pm on Wednesday, 24 August, while conducting a vehicle stop on a white Chevy Equinox.
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Officials soon noticed there were several black duffel bags in the back of the car, which was stopped near Gila Bend, Arizona.
They also said that the female driver was 'noticeably nervous' when questioned, and so they decided to conduct a full search.
After receiving her consent to look through the vehicle, officials found 'three bags containing packages' which were wrapped in cellophane and tape.
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All of the packages had been coated in axle grease, a technique which is said to often be used to mask the smell of drugs.
Upon further investigation, they uncovered 340 packages of fentanyl pills weighing 187 pounds and worth an estimated $4.3 million (£3.6 million).
The DEA said this is enough of the illegal substance to kill approximately 42,410,900 people.
Both the driver and her female passenger are facing drugs charges after the case was handed over to the Maricopa County Sheriff’s Office.
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Speaking about the discovery, Sheriff Paul Penzone said: "I am grateful for the incredible work by Border Patrol agents to keep drugs off our streets.
"Their efforts will save lives and promote safety."
US Customs and Border Protection went on to say that fentanyl seizures are a top priority due to the 'extreme danger' the narcotic poses to communities.
During a recent suspected smuggling incident, Tucson Sector agents had to administer naloxone to a man who displayed signs of an overdose.
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Fentanyl is strong opioid painkiller and only available on prescription.
Used to treat severe pain, for example during or after an operation or a serious injury, or pain from other diseases, the drug is up to 50 times stronger than heroin.
While common effects of the substance are euphoria, sleepiness and pain relief, the risks are hefty, from constipation and nausea to respiratory arrest and death, especially if mixed with other class A drugs.
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If you want friendly, confidential advice about drugs, you can talk to FRANK. You can call 0300 123 6600, text 82111 or contact through their website 24 hours a day, 365 days a year, or livechat from 2pm-6pm any day of the week
Daisy Phillipson