Complex sentence: EPD was available in the United States until 2001, when the Food and Drug Administration revoked approval for an investigative study which it had previously sanctioned which had allowed EPD to be imported into the USA without being licensed.
Split into simpler sentences: EPD was available in the United States until 2001, when the Food and Drug Administration revoked approval for an investigative study which it had previously sanctioned. That study had allowed EPD to be imported into the USA without being licensed.