The term 'psychoactive drug' is used to describe any chemical substance that affects mood, perception or consciousness as a result of changes in the functioning of the nervous system (brain and spinal cord). (http://www.Nt.Gov.Au/health/healthdev/health_promotion/bushbook/volume2/chap1/sect1.Htm)
If the brain records information based off of sensory information, a substance that alters perception, may be able to alter those perception there by causing false information to be recorded and when attempting to play back that information what actually happened will be written as the false memories of said events.
The drugs such as depressants, would cause memory disruption and slowed neural processing. When a person drinks enough to fall into blackout, he would not remember anything while he was drunk. Then he would assume what happened and would think that actually happened that time. With other drugs that plays with your memory, might do the similar things. How's your thoughts :-P ?