(CNN) In an effort to justify new tariffs on Mexico, President Donald Trump decried what he sees as the country's failure to stop the import of illicit drugs into the US via the southern border.

"90% of the Drugs coming into the United States come through Mexico & our Southern Border," Trump tweeted . "This has gone on for many years & nothing has been done about it."

It's important to note upfront that Trump's claim is not about the origin of where these drugs are manufactured, but rather where they enter the US. Much of the data is derived from drug seizure statistics, which provide a less-complete picture of where these drugs are coming from.

Facts First: While a majority of illicit drugs enter the US through the southern border when considering measurements like the purity of certain drugs, it's unlikely that Trump's 90% figure is accurate.

Due to the difference in the purity of particular drugs, it's difficult to measure what percentage of total drugs are coming from which country since the primary measure (gross weight) provides an incomplete picture. For instance, in the US Department of Justice Drug Enforcement Administration's 2018 National Drug Threat Assessment the agency notes that it is impossible to determine what country supplies the most fentanyl.

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