It looks like the overall picture is that nicotine is absorbed well in the intestines and the colon, but not so well in the stomach; this might be the explanation for the lack of effect, except on the other hand, the specific estimates I see are that 10-20% of the nicotine will be bioavailable in the stomach (as compared to 50%+ for mouth or lungs)… so any of my doses of >5ml should have overcome the poorer bioavailability! But on the gripping hand, these papers are mentioning something about the liver metabolizing nicotine when absorbed through the stomach, so…
A large review published in 2011 found that the drug aids with the type of memory that allows us to explicitly remember past events (called long-term conscious memory), as opposed to the type that helps us remember how to do things like riding a bicycle without thinking about it (known as procedural or implicit memory.) The evidence is mixed on its effect on other types of executive function, such as planning or ability on fluency tests, which measure a person’s ability to generate sets of data—for example, words that begin with the same letter.
Bacopa Monnieri: Also known as “waterhyssop,” this herb grows in wetlands around the world. It has a long history of use in Ayurvedic medicine. It is a powerful antioxidant which had demonstrated protective effects on cells. It also has anti-inflammatory properties. Inflammation is believed to play a major role in the development of dementia. Additionally, this herb boosts blood flow to the brain and activates choline acetyltransferase, a key enzyme which is necessary to synthesize the neurotransmitter cetylcholine.
When you start taking legit nootropics, you get to leave all of that behind you. You may never achieve perfect concentration (most of us never will), but you should find you are able to concentrate on the task at hand for much longer than you do now. You will end up taking fewer breaks, and you might start finishing up your work on time each day again—or even early!
One reason I like modafinil is that it enhances dopamine release, but it binds to your dopamine receptors differently than addictive substances like cocaine and amphetamines do, which may be part of the reason modafinil shares many of the benefits of other stimulants but doesn’t cause addiction or withdrawal symptoms.   It does increase focus, problem-solving abilities, and wakefulness, but it is not in the same class of drugs as Adderall, and it is not a classical stimulant. Modafinil is off of patent, so you can get it generically, or order it from India. It’s a prescription drug, so you need to talk to a physician.
Finally, it’s not clear that caffeine results in performance gains after long-term use; homeostasis/tolerance is a concern for all stimulants, but especially for caffeine. It is plausible that all caffeine consumption does for the long-term chronic user is restore performance to baseline. (Imagine someone waking up and drinking coffee, and their performance improves - well, so would the performance of a non-addict who is also slowly waking up!) See for example, James & Rogers 2005, Sigmon et al 2009, and Rogers et al 2010. A cross-section of thousands of participants in the Cambridge brain-training study found caffeine intake showed negligible effect sizes for mean and component scores (participants were not told to use caffeine, but the training was recreational & difficult, so one expects some difference).
Theanine can also be combined with caffeine as both of them work in synergy to increase memory, reaction time, mental endurance, and memory. The best part about Theanine is that it free of side effects and is easily available in the form of capsules. A natural option would be to use a good green tea brand which constitutes of tea grown in the shade, because then Theanine would be abundantly present in it.
The low-carb & high-fat diet (includes keto-diet) are not good for you because the brain needs glucose for fuel. It can burn fat. But, the brain’s preferred energy source is glucose. The key is to provide the brain with glucose without raising glucose/serum blood level. You do that by avoiding sugar and eating complex carbohydrates (fresh produce) that convert into glucose.
One claim was partially verified in passing by Eliezer Yudkowsky (Supplementing potassium (citrate) hasn’t helped me much, but works dramatically for Anna, Kevin, and Vassar…About the same as drinking a cup of coffee - i.e., it works as a perker-upper, somehow. I’m not sure, since it doesn’t do anything for me except possibly mitigate foot cramps.)
Nootropics (/noʊ.əˈtrɒpɪks/ noh-ə-TROP-iks) (colloquial: smart drugs and cognitive enhancers) are drugs, supplements, and other substances that may improve cognitive function, particularly executive functions, memory, creativity, or motivation, in healthy individuals. While many substances are purported to improve cognition, research is at a preliminary stage as of 2018, and the effects of the majority of these agents are not fully determined.