Harvard Medical School professor Bertha Madras said on Tuesday that the opioid crisis has its roots in the international drug trade, calling it an "epidemic of poisoning by international entities."

"The trans-shipment of fentanyl into our country has really made the opioid crisis far more dramatic, far more drastic than we've ever seen before," Madras, who serves on President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE's commission on combatting the opioid crisis, told Hill.TV's Buck Sexton and Krystal Ball on "Rising."

Madras said the issue began after the U.S. pharmaceutical industry pushed the idea that opioids could safely treat chronic pain.

"After that, when the controls began to tighten on the supply of opioids, there was a shift toward heroin, and then the producers in China began to produce a much more potent form [of] opioid, which is fentanyl," she continued. "Since 2013, it has been fentanyl that is developing the lion's share of causes of death."

"We now have an epidemic of poisoning by international entities, international drug cartels, and that's one of the biggest problems we have currently," she said.

Madras's comments come one day after the Senate passed a bipartisan package of 70 bills aimed at fighting the opioid epidemic.

The STOP Act, a major component of the package, would crack down on the shipment of deadly, synthetic opioids into the U.S. from other countries.

The U.S. Postal Service (USPS) has become a go-to shipper for drug traffickers looking to ship drugs into the U.S. because it is not required to gather data on incoming international shipments.

The legislation would require that USPS screen packages coming from overseas.

— Julia Manchester