What causes lingering after effects of most drugs?

[Synaps3]

I was just thinking about how if you take a moderate dosage of any drug, a lot of time even after double the half-life of the drug, you can still sense something in your system. My guess is that these lingering effects are either caused by the drug building up in fat tissue and getting released slowly over a period of time or maybe the receptors just get "stuck in the on position" after being on a drug for a while. What causes this?

 

[nAON]

After 2x half lives there's still 25% of the drug dose in your system, usually takes 6+ half lives for a drug to be considered 'cleared'. Though there could also be effects such as neurotransmitter depletion (eg. MDMA causes neurones to release serotonin, too much and they don't have enough to communicate normally) or the receptors desensitizing/resensitizing over time (ie. when a drug is hitting a group of receptors hard, the body tries to adjust back to 'normal' by withdrawing the receptors or making them inactive).