The evidence? Ritalin is FDA-approved to treat ADHD. It has also been shown to help patients with traumatic brain injury concentrate for longer periods, but does not improve memory in those patients, according to a 2016 meta-analysis of several trials. A study published in 2012 found that low doses of methylphenidate improved cognitive performance, including working memory, in healthy adult volunteers, but high doses impaired cognitive performance and a person’s ability to focus. (Since the brains of teens have been found to be more sensitive to the drug’s effect, it’s possible that methylphenidate in lower doses could have adverse effects on working memory and cognitive functions.)
Stimulants are the smart drugs 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 smart drugs 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.
With the right lifestyle and the right stack of supplements and nootropics, you can enjoy enhanced mental clarity, easier flow, and better vision. The best nootropics for your needs will depend on how much you want to spend, how often you want to take them, and what you want to take them for. Nutritional supplements should be taken daily, for the cumulative effect, but Smart drugs such as noopept and modafinil are usually taken on an as-needed basis, for those times when you are aiming for hyperfocus, better clarity, and better recall, or the ability to process a huge amount of incoming visual information quickly and accurately and to pick up on details that you might otherwise miss.
But notice that most of the cost imbalance is coming from the estimate of the benefit of IQ - if it quadrupled to a defensible $8000, that would be close to the experiment cost! So in a way, what this VoI calculation tells us is that what is most valuable right now is not that iodine might possibly increase IQ, but getting a better grip on how much any IQ intervention is worth.

3 days later, I’m fairly miserable (slept poorly, had a hair-raising incident, and a big project was not received as well as I had hoped), so well before dinner (and after a nap) I brew up 2 wooden-spoons of Malaysia Green (olive-color dust). I drank it down; tasted slightly better than the first. I was feeling better after the nap, and the kratom didn’t seem to change that.
On the other hand, sometimes you’ll feel a great cognitive boost as soon as you take a pill. That can be a good thing or a bad thing. I find, for example, that modafinil makes you more of what you already are. That means if you are already kind of a dick and you take modafinil, you might act like a really big dick and regret it. It certainly happened to me! I like to think that I’ve done enough hacking of my brain that I’ve gotten over that programming… and that when I use nootropics they help me help people.

When you drink tea, you’re getting some caffeine (less than the amount in coffee), plus an amino acid called L-theanine that has been shown in studies to increase activity in the brain’s alpha frequency band, which can lead to relaxation without drowsiness. These calming-but-stimulating effects might contribute to tea’s status as the most popular beverage aside from water. People have been drinking it for more than 4,000 years, after all, but modern brain hackers try to distill and enhance the benefits by taking just L-theanine as a nootropic supplement. Unfortunately, that means they’re missing out on the other health effects that tea offers. It’s packed with flavonoids, which are associated with longevity, reduced inflammation, weight loss, cardiovascular health, and cancer prevention.

It’s not clear that there is much of an effect at all. This makes it hard to design a self-experiment - how big an effect on, say, dual n-back should I be expecting? Do I need an arduous long trial or an easy short one? This would principally determine the value of information too; chocolate seems like a net benefit even if it does not affect the mind, but it’s also fairly costly, especially if one likes (as I do) dark chocolate. Given the mixed research, I don’t think cocoa powder is worth investigating further as a nootropic.

Probably most significantly, use of the term “drug” has a significant negative connotation in our culture. “Drugs” are bad: So proclaimed Richard Nixon in the War on Drugs, and Nancy “No to Drugs” Reagan decades later, and other leaders continuing to present day. The legitimate demonization of the worst forms of recreational drugs has resulted in a general bias against the elective use of any chemical to alter the body’s processes. Drug enhancement of athletes is considered cheating – despite the fact that many of these physiological shortcuts obviously work. University students and professionals seeking mental enhancements by taking smart drugs are now facing similar scrutiny.
Some of the newest substances being used as ‘smart drugs’ are medically prescribed for other conditions. For example, methylphenidate, commonly known as Ritalin, is used to treat attention deficit hyperactivity disorder (ADHD). So is Adderall, a combination drug containing two forms of amphetamine. These are among a suite of pharmaceuticals now being used by healthy people, particularly university students, to enhance their capabilities for learning or working. 

This mental stimulation is what increases focus and attention span in the user. The FDA permitted treatments for Modafinil include extreme sleepiness and shift work disorder. It can also get prescribed for narcolepsy, and obstructive sleep apnea. Modafinil is not FDA approved for the treatment of ADHD. Yet, many medical professionals feel it is a suitable Adderall alternative.
See Melatonin for information on effects & cost; I regularly use melatonin to sleep (more to induce sleep than prolong or deepen it), and investigating with my Zeo, it does seem to improve & shorten my sleep. Some research suggests that higher doses are not necessarily better and may be overkill, so each time I’ve run out, I’ve been steadily decreasing the dose from 3mg to 1.5mg to 1mg, without apparently compromising the usefulness.
Omega-3 fatty acids: DHA and EPA – two Cochrane Collaboration reviews on the use of supplemental omega-3 fatty acids for ADHD and learning disorders conclude that there is limited evidence of treatment benefits for either disorder.[42][43] Two other systematic reviews noted no cognition-enhancing effects in the general population or middle-aged and older adults.[44][45]