I previously discussed human development from conception to adulthood, because it’s relevant to a fundamental question – What can we change about ourselves after we're born (using drugs, or by altering our genes, etc)?
Through the process of development, we're born with two arms and two legs, and it seems nearly impossible to add another arm or leg (with drugs, or otherwise), since the differentiation of cells takes place in irreversible stages, and quite early in the process. Those development genes are programmed to work in the context of an undeveloped body, not a fully developed one.
Still, some things can be changed after we're fully developed. Some psychological outcomes (like depression) can be treated by drugs. But it’s not clear that the underlying cause is treatable. If you get depressed easily if someone dies, that tendency is deeply embedded in the developed circuitry of your brain. Other people are less depressed in the same situation, due to their gene variants, which program their brains to be less sensitive to traumatic events.
There is some hope that stem cells could be introduced into an adult body, and re-generate cells (nerves, skin, etc). Skin can repair itself when cut, because we all have stem cells in our skin. So some development is possible, even after we've reached adulthood. But unless that new development is guided differently from before (by different genes), it won’t make you a different person.
Drugs can affect us, but only to the degree that the form of the body has developed itself to be affected. Recent debates have focused on how many targets (usually proteins expressed by genes) in the human body are druggable. Possibly 3,000 to 5,000 targets are druggable, which is only a tiny percentage of the whole.
Any change to our genes would require a change to the genes in each of our 100,000,000,000,000 body cells, which is certainly possible, through the introduction of HIV-like viruses, which can embed new genes inside many of our rapidly dividing cells. But even new genes can't reverse all the previous steps of development and re-generate our brains from scratch.
So our personality probably can't be affected by either drugs or gene treatments. Certainly, we know of targets in the brain that allow us to mitigate conditions like depression, and we have antidepressant drugs for that. But do targets exist to improve charisma, ambition, or desire for leadership? I doubt it. Does the development of the brain leave itself open, such that we may alter our basic personality with drugs? I don't think so. So the only time we can dramatically change things (personality, etc) is at the time of conception.