But, thanks to the efforts of a number of remarkable scientists, researchers and plain-old neurohackers, we are beginning to put together a “whole systems” model of how all the different parts of the human brain work together and how they mesh with the complex regulatory structures of the body. It’s going to take a lot more data and collaboration to dial this model in, but already we are empowered to design stacks that can meaningfully deliver on the promise of nootropics “to enhance the quality of subjective experience and promote cognitive health, while having extremely low toxicity and possessing very few side effects.” It’s a type of brain hacking that is intended to produce noticeable cognitive benefits.
Absorption of nicotine across biological membranes depends on pH. Nicotine is a weak base with a pKa of 8.0 (Fowler, 1954). In its ionized state, such as in acidic environments, nicotine does not rapidly cross membranes…About 80 to 90% of inhaled nicotine is absorbed during smoking as assessed using C14-nicotine (Armitage et al., 1975). The efficacy of absorption of nicotine from environmental smoke in nonsmoking women has been measured to be 60 to 80% (Iwase et al., 1991)…The various formulations of nicotine replacement therapy (NRT), such as nicotine gum, transdermal patch, nasal spray, inhaler, sublingual tablets, and lozenges, are buffered to alkaline pH to facilitate the absorption of nicotine through cell membranes. Absorption of nicotine from all NRTs is slower and the increase in nicotine blood levels more gradual than from smoking (Table 1). This slow increase in blood and especially brain levels results in low abuse liability of NRTs (Henningfield and Keenan, 1993; West et al., 2000). Only nasal spray provides a rapid delivery of nicotine that is closer to the rate of nicotine delivery achieved with smoking (Sutherland et al., 1992; Gourlay and Benowitz, 1997; Guthrie et al., 1999). The absolute dose of nicotine absorbed systemically from nicotine gum is much less than the nicotine content of the gum, in part, because considerable nicotine is swallowed with subsequent first-pass metabolism (Benowitz et al., 1987). Some nicotine is also retained in chewed gum. A portion of the nicotine dose is swallowed and subjected to first-pass metabolism when using other NRTs, inhaler, sublingual tablets, nasal spray, and lozenges (Johansson et al., 1991; Bergstrom et al., 1995; Lunell et al., 1996; Molander and Lunell, 2001; Choi et al., 2003). Bioavailability for these products with absorption mainly through the mucosa of the oral cavity and a considerable swallowed portion is about 50 to 80% (Table 1)…Nicotine is poorly absorbed from the stomach because it is protonated (ionized) in the acidic gastric fluid, but is well absorbed in the small intestine, which has a more alkaline pH and a large surface area. Following the administration of nicotine capsules or nicotine in solution, peak concentrations are reached in about 1 h (Benowitz et al., 1991; Zins et al., 1997; Dempsey et al., 2004). The oral bioavailability of nicotine is about 20 to 45% (Benowitz et al., 1991; Compton et al., 1997; Zins et al., 1997). Oral bioavailability is incomplete because of the hepatic first-pass metabolism. Also the bioavailability after colonic (enema) administration of nicotine (examined as a potential therapy for ulcerative colitis) is low, around 15 to 25%, presumably due to hepatic first-pass metabolism (Zins et al., 1997). Cotinine is much more polar than nicotine, is metabolized more slowly, and undergoes little, if any, first-pass metabolism after oral dosing (Benowitz et al., 1983b; De Schepper et al., 1987; Zevin et al., 1997).

But when aficionados talk about nootropics, they usually refer to substances that have supposedly few side effects and low toxicity. Most often they mean piracetam, which Giurgea first synthesized in 1964 and which is approved for therapeutic use in dozens of countries for use in adults and the elderly. Not so in the United States, however, where officially it can be sold only for research purposes.

By the way, since I’ll throw around the term a few more times in this article, I should probably clarify what an adaptogen actually is. The actual name adaptogen gives some hint as to what these fascinating compounds do: they help you to adapt, specifically by stimulating a physiological adaptive response to some mild, hormesis-like stressor. A process known as general adaptation syndrome (GAS) was first described by the 20th-century physician and organic chemist Hans Selye, who defined GAS as the body’s response to the demands placed upon it. When these demands are excessive and consistent, it can result in the common deleterious symptoms now associated with long-term exposure to chronic stress. GAS is comprised of an alarm stage (characterized by a burst of energy), a resistance stage (characterized by resistance or adaptation to the stressor), and – in the case of excessive and chronic stress – an exhaustion stage (characterized by energy depletion). Adaptogens are plant-derived compounds capable of modulating these phases of GAS by either downregulating stress reactions in the alarm phase or inhibiting the onset of the exhaustion phase, thus providing some degree of protection against damage from stress.
A fancier method of imputation would be multiple imputation using, for example, the R library mice (Multivariate Imputation by Chained Equations) (guide), which will try to impute all missing values in a way which mimicks the internal structure of the data and provide several possible datasets to give us an idea of what the underlying data might have looked like, so we can see how our estimates improve with no missingness & how much of the estimate is now due to the imputation:
He first took up the game in 1995, when he was in college. He recalled: "It was very mathematical, but you could also inject yourself into the game and manipulate the other guy with words" - more so than in a game like chess. Phillips soon felt that he had mastered the strategic aspects of poker. The key variable was execution. At tournaments he needed to be able to stay focused for 14 hours at a stretch, often for several days, but he found it difficult to do so. In 2003, a doctor gave him a diagnosis of ADHD and he began taking Adderall. Within six months, he had won $1.6m at poker - far more than he'd won in the previous four years. Adderall not only helped him concentrate, it also helped him resist the impulse to keep playing losing hands out of boredom. In 2004, Phillips asked his doctor to give him a prescription for Provigil, which he added to his Adderall regimen. He took 200-300mg of Provigil a day, which he felt helped him settle into an even more serene and objective state of mindfulness; as he put it, he felt "less like a participant than an observer - and a very effective one". Though Phillips sees neuroenhancers as essentially steroids for the brain, they haven't yet been banned from poker competitions.
In fact, this body-mind connection has become so relevant to our current era that communities like Mental Health America are devoting their efforts to create a challenge that raises awareness on how lifestyle plays an important role on our mental health. While our generation is definitely more conscious of our bodies and the importance of a healthy lifestyle, it's a good reminder that the body is like a machine and we should listen to it, tune it up, and update the system every so often. 
Racetams, such as piracetam, oxiracetam, and aniracetam, which are often marketed as cognitive enhancers and sold over-the-counter. Racetams are often referred to as nootropics, but this property is not well established.[31] The racetams have poorly understood mechanisms, although piracetam and aniracetam are known to act as positive allosteric modulators of AMPA receptors and appear to modulate cholinergic systems.[32]