Drugs targeting the central nervous system take significantly longer to develop and get approved than other meds, according to a new analysis.

Tufts University’s Center for Drug Development analyzed 509 drugs and biologics tested and approved since 2000 and found that CNS drugs took 20 percent longer to develop and about 19 months, or 36 percent longer on average than other meds, to win FDA approval after clinical trials.

CNS drugs were approved at a 6 percent faster clip than other drugs between 2012 and 2017, according to the report, but it’s unclear whether that trend will last.