"Another point worth keeping in mind is that there tends to be a continuous flow of new drugs onto the scene and of older ones being rediscovered by young people. Many drugs have made a comeback years after they first fell from popularity, often because knowledge among youth of their adverse consequences faded as generational replacement took place. We call this process 'generational forgetting.' Examples include LSD and methamphetamine, two drugs used widely in the 1960s that made a comeback in the 1990s after their initial popularity faded as a result of their adverse consequences becoming widely recognized during periods of high use. Heroin, cocaine, PCP, and crack are some others that have followed a similar pattern. LSD, inhalants, and ecstasy have all shown some effects of generational forgetting in recent years — that is, perceived risk has declined appreciably for those drugs — which puts future cohorts at greater risk of having a resurgence in use. In the case of LSD, perceived risk among 8th graders has declined noticeably, and more students are saying that they are not familiar with the drug. It would appear that a resurgence in availability (which declined very sharply after about 2001, most likely due to the FDA closing a major lab in 2000) could generate another increase in use."
Johnston, L. D., O’Malley, P. M., Miech, R. A., Bachman, J. G., & Schulenberg, J. E. (2014). Monitoring the Future national results on drug use: 1975-2013: Overview, Key Findings on Adolescent Drug Use. Ann Arbor: Institute for Social Research, The University of Michigan, p. 49.