I’ve tracked the claim to the original source. “It is reported that in underdeveloped countries such as Argentina, Colombia, and Mexico, up to 40% of manufactured pharmaceuticals may be counterfeit.” Note that the source of the drugs or the type of drugs is not mentioned at all in the report.
Despite being quoted all over the internet, this is a brief stat in an FDA report from 2003:
https://permanent.access.gpo.gov/LPS112182/LPS112182_interim_report.pdf. The report doesn’t mention Mexico again… so it’s difficult to tell how they determined that statistic.
For people who are interested in purchasing drugs from Mexico, I’ve listed a couple guides below that can help you purchase them more safely. Buying from a licensed pharmacy is the number one recommendation across all articles I read.
Also, I found this dissertation interesting: https://dash.harvard.edu/bitstream/handle/1/8965600/Lazar06.pdf?sequence=1