Does legal marijuana lead to the use of more dangerous drugs, or increase crime?

 It appears that the short answer is "No," according to this recent NBER working paper

Is Recreational Marijuana a Gateway to Harder Drug Use and Crime?  by Joseph J. Sabia, Dhaval M. Dave, Fawaz Alotaibi & Daniel I. Rees

WORKING PAPER 29038, DOI 10.3386/w29038,  July 2021

Recreational marijuana laws (RMLs), which legalize the possession of small quantities of marijuana for recreational use, have been adopted by 18 states and the District of Columbia. Opponents argue that RML-induced increases in marijuana consumption will serve as a “gateway” to harder drug use and crime. Using data covering the period 2000-2019 from a variety of national sources (the National Survey of Drug Use and Health, the Uniform Crime Reports, the National Vital Statistics System, and the Treatment Episode Data Set) this study is the first to comprehensively examine the effects of legalizing recreational marijuana on hard drug use, arrests, drug overdose deaths, suicides, and treatment admissions. Our analyses show that RMLs increase adult marijuana use and reduce drug-related arrests over an average post-legalization window of three to four years. There is little evidence to suggest that RML-induced increases in marijuana consumption encourage the use of harder substances or violent criminal activity.