Harrison, 39, who will be entering his 14th season, used Instagram to post a picture of a test sample while pointing out in the caption that he's received three random tests in the past three months.
NFL players can be tested up to six times in an offseason, according to the collective bargaining agreement, but Harrison has suggested in the past that he's been unfairly targeted and has been known to joke about the league's drug-testing tendencies.
Harrison's agent, Bill Parise, said Harrison is tested frequently, but "they have the right to test him and we understand that."
Harrison, who has continued to perform at a high level, also was tested multiple times at the end of the 2016 season.
League testing, done by an independent administrator, uses a computer program to randomly select players for samples. The NFL doesn't have access to that system, which can test 10 players each week.
Only players under the "reasonable cause" list -- such as flunking a drug test in college, for example -- get targeted.
"I'm starting to think it's not as random," Harrison told Pennlive.com in December 2016. "I just wish I had that much luck with the lottery. That's all."
Harrison was one of five players investigated by the league last offseason over an Al-Jazeera America report alleging performance-enhancing drug use. The source of the story recanted, and the league found no wrongdoing.