Funding for this research was provided by:
National Institute on Drug Abuse (R01DA03889, T32 DA07209)
Received: 2 March 2020
Accepted: 22 April 2020
First Online: 5 June 2020
Compliance with ethical standards
: The study was conducted in compliance with US laws.
: Roland Griffiths is on the Board of Directors of the Heffter Research Institute.