2 commenters point out that my possible lack of result is due to my mistaken assumption that if nicotine is absorbable through skin, mouth, and lungs it ought to be perfectly fine to absorb it through my stomach by drinking it (rather than vaporizing it and breathing it with an e-cigarette machine) - it’s apparently known that absorption differs in the stomach.
The single most reliable way to protect our brain cells as we age, most researchers agree, is to eat plenty of fruits and vegetables, which are chock-full of antioxidants and nutrients. In a study published in the October 1997 issue of the American Journal of Clinical Nutrition, researchers tested 260 people aged 65 to 90 with a series of mental exercises that involved memorizing words or doing mental arithmetic. The top performers were those who consumed the most fruits and vegetables and ate the least artery-clogging saturated fat.

Vitamin D is probably the most important supplement you can take, and one of the best brain food. It acts on more than over 1,000 different genes and is a substrate for testosterone, progesterone, estradiol, and other  hormones.[1] It also influences inflammation and brain calcium absorption.[2] No surprise that optimal vitamin D levels are linked to stronger cognitive function and slower brain aging.[3][4]
I had tried 8 randomized days like the Adderall experiment to see whether I was one of the people whom modafinil energizes during the day. (The other way to use it is to skip sleep, which is my preferred use.) I rarely use it during the day since my initial uses did not impress me subjectively. The experiment was not my best - while it was double-blind randomized, the measurements were subjective, and not a good measure of mental functioning like dual n-back (DNB) scores which I could statistically compare from day to day or against my many previous days of dual n-back scores. Between my high expectation of finding the null result, the poor experiment quality, and the minimal effect it had (eliminating an already rare use), the value of this information was very small.
And when it comes to your brain, it’s full of benefits, too. Coconut oil works as a natural anti-inflammatory, suppressing cells responsible for inflammation. It can help with memory loss as you age and destroy bad bacteria that hangs out in your gut. (5) Get your dose of coconut oil in this Baked Grouper with Coconut Cilantro Sauce or Coconut Crust Pizza.
Low-dose lithium orotate is extremely cheap, ~$10 a year. There is some research literature on it improving mood and impulse control in regular people, but some of it is epidemiological (which implies considerable unreliability); my current belief is that there is probably some effect size, but at just 5mg, it may be too tiny to matter. I have ~40% belief that there will be a large effect size, but I’m doing a long experiment and I should be able to detect a large effect size with >75% chance. So, the formula is NPV of the difference between taking and not taking, times quality of information, times expectation: \frac{10 - 0}{\ln 1.05} \times 0.75 \times 0.40 = 61.4, which justifies a time investment of less than 9 hours. As it happens, it took less than an hour to make the pills & placebos, and taking them is a matter of seconds per week, so the analysis will be the time-consuming part. This one may actually turn a profit.
Methylphenidate – a benzylpiperidine that had cognitive effects (e.g., working memory, episodic memory, and inhibitory control, aspects of attention, and planning latency) in healthy people.[21][22][23] It also may improve task saliency and performance on tedious tasks.[25] At above optimal doses, methylphenidate had off–target effects that decreased learning.[26]