WASHINGTON (REUTERS) - The US Justice Department sued Walmart on Tuesday (Dec 22), accusing the retailer of fuelling the opioid crisis in the United States, ignoring warning signs from its pharmacists and filling thousands of invalid prescriptions.

In a civil lawsuit filed in US District Court in Delaware, the department accused Walmart of failing to take its gatekeeping duties as a pharmacy seriously.

Walmart, the world's biggest retailer, created a system that turned its 5,000 in-store pharmacies into a supplier of highly addictive painkillers, dating as early as June 2013, the lawsuit said.

The retailer's shares were trading down 1.5 per cent following the news.

Walmart's "unlawful" actions helped "fuel a national crisis" and had "disastrous consequences," said Jeffrey Bossert Clark, the acting head of the Justice Department's civil division at a press briefing.

Asked if the government was planning on bringing criminal charges, Clark said "you should not draw any inferences about any criminal matters" from the civil filing.

Walmart declined to immediately comment on the lawsuit.

The opioid epidemic has claimed the lives of roughly 450,000 people across the United States since 1999 due to overdoses from prescription painkillers and illegal drugs.

According to the lawsuit, Walmart "unlawfully filled thousands upon thousands of invalid controlled-substance prescriptions."

The lawsuit said that "for years, Walmart kept in place a system that it knew was failing to adequately detect and report suspicious orders."

It accused Walmart of violating the Controlled Substances Act.

If found liable, it could face civil penalties of up to US$67,627 (S$90,000) for each unlawful prescription filled and US$15,691 for each suspicious order not reported.

"Walmart managers put enormous pressure on pharmacists to fill prescriptions" and required them to process a high volume of prescriptions "as fast as possible" while denying them the authority to refuse to fill what they knew to be invalid prescriptions, the government said.

In October, Walmart filed a lawsuit in Texas against the federal government seeking clarity on the roles and legal responsibilities of pharmacists and pharmacies in filling opioid prescriptions.

Walmart said in October that federal authorities were seeking civil penalties related to allegations it failed to submit suspicious order reports and said that this potential move would be "unprecedented."