Intelligence is selected for in humans. Therefore any individual gene that significantly boosts intelligence should be expected to have already spread through t

Intelligence is selected for in humans. Therefore any individual gene that significantly boosts intelligence should be expected to have already spread through the population, or has a trade-off that reduces reproductive fitness. The same can be said with drugs -- any chemical that your body could have reasonably produced on its own should be expected to reduce your reproductive fitness. So brain drugs should nearly all have side-effects.