Being stoned does not necessarily make you a bad eyewitness.
A recent study by Vredeveldt et al has shown that witnesses to a crime could pick the thief out of a line up just as well if they had been smoking marijuana as if they had not.
Participants were asked, either before or after consuming marijuana in a coffee shop in Amsterdam, to watch a video of a robbery. Then, after a short interval, they had to describe what they had seen, and try to say whether the robber was in present in a police lineup.
The more stoned participants were not so good at giving details of the crime generally, which might make them less well believed by judges and juries. However, they were as good as sober participants at identifying whether the thief was in the lineup or not. Curiously, the intoxicated ones were more likely to be correct if they were confident than the sober ones were if they were confident.
This is only one study, and it is not perfect. For example, the people were presumably all consumers of marijuana generally as they were going into a coffee shop which expressly offers this service. Some of the participants who were deemed sober because they were going in may therefore already have partaken somewhere else; or there may be an overall long term effect of the marijuana which would have affected all participants.
And the list of possible flaws goes on. However, from this study, it appears that being high on dope doesn’t necessarily affect your natural recognition abilities. As cannabis is made legal in more and more places around the world, it is important that studies are undertaken to discover the effects and implications this will have on society.