One thing picked up from my Chemistry degree that didn't get washed away by years of gleeful boozing is the following:

The synthesis of many drugs uses strychnine or brucine as resolving agents to split the products into their "left-handed" and "right-handed" forms. Having used them, it's not easy to completely eliminate traces of the resolving agent from the final batch - and as you can imagine, that job's usually not done with quite the same care and attention in an illegal drug lab as in a well-stocked pharmaceutical company. The end result is that there's often enough traces of strychnine or brucine in synthetic drugs like MDMA or LSD to cause cramping.

(The Organic Synthesis course used LSD and cocaine as its two main examples, on the grounds that students were more likely to pay attention to something that looked a bit naughty)

There are a lot more, and some interesting ones that have recently been created in the lab such as Bromo-Dragonfly which is active at 500 micrograms. This is a potent hallucinogen. There is proof of Bromo-Dragonfly being on blotter paper on the DEA web site.

What one society or group or individual takes for granted as self-evidently harmful, others view as obviously beneficial, even necessary. In crucial ways, the issue of harm or danger to society as a result of the drug pivots on moot points, totally unanswerable questions, questions that science is unable to answer without the resolution of certain basic issues. And for many crucially debated marijuana questions, this modest requirement cannot be met. In other words, before we raise the question of whether marijuana has a desirable or a noxious effect, we must first establish the desirability or the noxiousness to whom.

The effects of marijuana on the human brain are numerous and complex. The psychoactive constituent of cannabis, delta-9 THC, affects the brain in a fundamentally different way than many other drugs, such as cocaine and opiates. These drugs are associated with the dopamine pathways within the brain and are subject to high rates of abuse, as seen with self-administration experiments done with animals.

Early research on cannabis' effects on the brain was limited by the technology of the time and was thus subject to influence by observations of behavior, rather than neuropharmacology. This lack of concrete knowledge about THC's effects allowed cultural biases to maintain fallacy that marijuana acted similarly to heroin. Research done during the past decade, since the discovery of a THC receptor in the brain, has refuted such claims. The congressional Office of Technology Assessment found that recent research findings, especially those done by Dr. Miles Herkenham of the National Institute of Mental Health (NIMH), have proved that marijuana has no effect on the dopamine-related brain systems.

Dopamine is a neurotransmitter in the brain that is associated with pleasure. The neural systems that are associated with dopamine are known as the "brain reward system." These systems are intimately involved with limbic system, an area of the brain which is associated with the control of emotions and behavior. Highly addictive drugs, such as cocaine, affect these pathways and cause an effective increase in the amount of dopamine in the brain. Cocaine and amphetamines block the reabsorbtion of dopamine, thus prolonging and intensifying the effects. Opiates activate production of dopamine by blocking the inhibitory signal, gamma-aminobutyric acid, which would ordinarily slow or stop dopamine production.

* Flashbacks. Way after the experience you can experience flashback that will suddenly put you back into the trip.