When researchers mapped the proportion of treatment episodes that lasted at least six months in each group, they discovered another gap: Starting in 2011, all racial groups were experiencing an increase in the percentage until 2016, when the percentage among Black patients reversed.

The researchers struggled to give explanations for those pivot points in interviews, saying that it would require speculation or “a carefully controlled analysis, which clearly goes beyond this short descriptive analysis,” said Huiru Dong, a co-author and postdoctoral research associate at Harvard Medical School.

Outside researchers, however, were happy to hypothesize. For example, Medicaid expanded in 2014, and access to buprenorphine disproportionately increased in ZIP codes with higher percentages of white residents, according to Dr. Bradley Stein, the director of the RAND Opioid Policy Center and a senior physician policy researcher at the RAND Corporation.

Dr. Jordan, the addiction researcher at N.Y.U., attributed racial disparities in treatment duration to an array of factors, including physician bias and even buprenorphine marketing strategies.

Optimal length of buprenorphine treatment is a source of “very lively, active debate” among clinicians and researchers, said Erin Stringfellow, one of the study’s co-authors. Some urge patients to eventually wean off the opioid, but others believe it is best to continue a medication-assisted treatment plan indefinitely.

Still, the minimum recommended duration for buprenorphine treatment is six months, so all experts agreed: The treatment figures for all groups in 2020, the most recent available year, were far too low.

For white patients, the median treatment duration was about 53 days; for Black patients, about 44 days; and for Hispanic patients, less than 40.