Can't seem to recall where I read this…
But seriously - if you smoke pot, use hash, or whatever, while you are under its effect your short-term memory doesn't work properly - the neurons just don't connect the same way. While this has no long-range effect, it means you may have trouble remembering what you did while you were smoking, during and after the substance's effect.
When preparing for activities that require an operational short-term memory, consider the amount of time spent stoned recently. One unofficial guideline is to spend one day straight for each day spent stoned - e.g., if you've smoked two days in a row, spend two days straight. When preparing for exams after steadily smoking through the semester, take two weeks off smoking at the very least to recover your memory. Consider devising tests for your short-term memory during this time - the realisation that you are able to remember things again can be quite a significant experience.

While it is true that it's difficult to remember stoned activities while straight, it has also been noted that it's hard to remember things that happened while you weren't smoking while stoned, whereas previous sessions can be recalled with disproportionate ease. In addition to these effects, the stoned mind will sometimes recall dreams more easily, although this is a more subtle effect.

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