There are seven primary classes used to categorize nootropic drugs: Racetams, Stimulants, Adaptogens, Cholinergics, Serotonergics, Dopaminergics, and Metabolic Function Nootropics. Despite considerable overlap and no clear border in the brain and body's responses to these substances, each class manifests its effects through a different chemical pathway within the body.
Racetams are the best-known smart drugs on the market, and have decades of widespread use behind them. Piracetam is a leading smart drug, commonly prescribed to seniors with Alzheimer's or pre-dementia symptoms - but studies have shown Piracetam's beneficial effects extend to people of all ages, as young as university students. The Racetams speed up chemical exchange between brain cells. Effects include increases in verbal learning, mental clarity, and general IQ. Other members of the Racetam family include Pramiracetam, Oxiracetam, аnԁ Aniracetam, which differ from Piracetam primarily in their potency, not their actual effects.
Studies have shown that using racetams can deplete the brain of choline and cause headaches. To counteract this effect, users tend to take a choline supplement - generally choline bitartrate - in proportion to the amount of the racetam in their smart drug regimen.
The Racetams are not controlled substances in the US or most of the world, and can be purchased without prescription as over-the-counter dietary supplements. "Overdosing," per se, isn't really possible on these extremely non-toxic supplements, with the most extreme side effects felt by users being similar to drinking excessive caffeine.
Stimulants are the nootropics most familiar to people, starting with widely-used psychostimulants caffeine and nicotine, and the more ill-reputed subclass of amphetamines. Stimulant drugs generally function as nootropics in the sense that they promote general wakefulness and put the brain and body "on alert" in a ready-to-go state. Basically, any drug whose effects reduce drowsiness will increase the functional IQ, so long as the user isn't so over-stimulated they're shaking or driven to distraction.
The stimulant now most popular in news articles as a legitimate "smart drug" is Modafinil, which came to market as an anti-narcolepsy drug, but gained a following within the military, doctors on long shifts, and college students pulling all-nighters who needed a drug to improve alertness without the "wired" feeling associated with caffeine. Modafinil is a relatively new nootropic, having gained widespread use only in the past 15 years. More research is needed before scientists understand this drug's function within the brain - but the increase in alertness it provides is uncontested.
Adaptogens are plant-derived chemicals whose activity helps the body maintain or regain homeostasis (equilibrium between the body's metabolic processes). Almost without exception, adaptogens are available over-the-counter as dietary supplements, not controlled drugs. Well-known adaptogens include Ginseng, Kava Kava, Passion Flower, St. Johns Wort, and Gotu Kola. Many of these traditional remedies border on being "folk wisdom," and have been in use for hundreds or thousands of years, and are used to treat everything from anxiety and mild depression to low libido. While these nootropics work in a many different ways (their commonality is their resultant function within the body, not their chemical makeup), it can generally be said that the cognitive boost users receive is mostly a result of fixing an imbalance in people with poor diets, body toxicity, or other metabolic problems, rather than directly promoting the growth of new brain cells or neural connections.
The choline-based class of nootropics play important cognitive roles in memory, attention, and mood regulation. Acetylcholine (ACh) is one of the brain's primary neurotransmitters, and also vital in the proper functioning of the peripheral nervous system. Studies with rats have shown that certain forms of learning and neural plasticity seem to be impossible in acetylcholine-depleted areas of the brain. This is particularly worth mentioning because (as noted above under the Racetams section), the Racetam class of nootropics tends to deplete cholines from the brain, so one of the classic "supplement stacks" - chemical supplements that are used together - are Piracetam and Choline Bitartrate. Cholines can also be found in normal food sources, like egg yolks and soybeans.
Another well-known smart drug classed as a cholinergic is Sulbutiamine, a synthetic derivative of thiamine which crosses the blood-brain barrier and has been shown to improve memory while reducing psycho-behavioral inhibition. While Sulbutiamine has been shown to exhibit cholinergic regulation within the hippocampus, the reasons for the drug's discernable effects on the brain remain unclear. This nootropic, available over the counter as a nutritional supplement, has a long history of use, and appears to have no serious side effects at therapeutic levels.
Serotonin, or 5-hydroxytryptamine (5-HTP), is another primary neurotransmitter and controls major features of the mental landscape including mood, sleep and appetite. Serotonin is produced within the body by exposure, which is one reason that the folk-remedy of "getting some sun" to fight depression is scientifically credible. Many foods contain natural serotonergic (serotonin-promoting or releasing) compounds, including the well-known chemical L-Tryptophan found in turkey, which can promote sleep after big Thanksgiving dinners.
Iluminal is an example of an over-the-counter serotonergic drug used by people looking for performance enhancement, memory improvements, and mood-brightening. Also noteworthy, a wide class of prescription anti-depression drugs are based on serotonin reuptake inhibitors that slow the absorption of serotonin by the presynaptic cell, increasing the effect of the neurotransmitter on the receptor neuron - essentially facilitating the free flow of serotonin throughout the brain.
Dopaminergics are smart drug substances that affect levels of dopamine within the brain. Dopamine is a major neurotransmitter, responsible for the good feelings and biochemical positive feedback from behaviors for which our biology naturally rewards us: tasty food, sex, positive social relationships, etc. Use of dopaminergic nootropics promotes attention and alertness by either increasing the efficacy of dopamine within the brain, or inhibiting the enzymes that break dopamine down. Examples of popular dopaminergic nootropic drugs include Yohimbe, selegiline and L-Tyrosine.
Sulbutiamine, mentioned earlier as a cholinergic smart drug, can also be classed a dopaminergic, although its mechanism is counterintuitive: by reducing the release of dopamine in the brain's prefrontal cortex, the density of dopamine receptors actually increase after continued Sulbutiamine exposure, through a compensatory mechanism. (This provides an interesting example of how dividing nootropics into sensible "classes" is a matter of taste as well as science, especially since many of them create their discernable neural effects through still undefined mechanisms.)
Metabolic Function Nootropics
Metabolic function nootropics provide mental benefits by generally facilitating the body's metabolic processes related to the production of new tissues and the release of energy from food and fat stores. Creatine, a long-time favorite performance-enhancement drug for competitive athletes, was in the news recently when it was found in a double-blind, placebo-controlled crossover trial to have significant cognitive benefits - including both general speed of cognition and improvements in working memory. Ginkgo Biloba is another metabolic function nootropic used to increase memory and improve circulation - however, news from recent studies raises questions about these purported effects.