Basically it is when you have taken a substance & you can get flash back's these can occure much later after having taken the substance even years this can happen even if you have taken small amounts. It can even be years after taking the drug and no you don't have any trace of the drug in your system it is a relapse, so you experience the same feeling, emotions and perceptions as you would if you had taken the drug one hour ago.

If your experience was a bad or you had a bad trip it can be very disorienting if you are out shopping and the cans of beans start chasing you trying to eat you.