How far into your education are you?- If you are 1-2 years in I would suggest picking up a minor/double major in physiology or biology. Also you should start volunteering as a research assistant as soon as possible. My uni offers credit for the first 2 semesters of research assistant work, a pretty good deal as an undergrad (I started my senior year, some of my colleagues started their sophomore year). This is the best way to get work experience and will put you ahead of the rest for grad school admissions. It's also the best way to find out how research is ACTUALLY done, as well as introducing you to the wonderful world of academic red tape.
Cognitive Neuroscience is probably what you're looking for, may I ask what sorts of psychoactive drugs you are interested in? If you're thinking about strong psychedelic compounds, you would be wise to go with psychopharmocology as it will help you with credibility and what you need to do to make sure scientific requirements are met (every experiment you run has to be approved by an IRB first). But if you are interested in nootropics like the racetams, I think cognitive neuroscience would be your best bet.
Also, what are the benefits of a full education for you? (Just curious) Grad school is pretty heavy, if you don't dedicate your entire life to it, you won't make it through. I've been juggling with the thought of just shooting for a master's. You can get a job with a masters and 2 years instead of 5 years of education could be the difference between baldness and semi baldness

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Edit: As to what AminoAcid said about having a lot of chemistry experience: I wouldn't worry too much about that, you can always collaborate with someone who has a strong chemistry background.