Three Republican senators on Wednesday introduced a bill that would restrict fentanyl and another powerful opioid, carfentanil, from entering the United States.

Fentanyl has been linked to an increasing number of overdoses and deaths. Carfentanil, used as an elephant tranquilizer, has been blamed for a number of overdoses in Ohio, The Hill reports.

Some drug traffickers are using the U.S. mail system to illegally import the opioids from China and India. The new bill, called the Synthetics Trafficking and Overdose Prevention (STOP) Act, would strengthen standards for foreign postal services. The bill, introduced by Senators Rob Portman of Ohio, Ron Johnson of Wisconsin and Kelly Ayotte of New Hampshire, would require the same types of electronic data that private carriers such as FedEx or UPS provide.