The subfields most concerned with drugs, psychopharmacology or molecular psychiatry, are usually a branch of psychology programs, and deal with the molecular basis of the behavioral aspects of drugs.
But be advised, all the interesting courses aren't until graduate school usually; you need to choose an undergraduate program that prepares you for it. Neuroscience/psychobiology if your school has it, if not in most schools, there's a BA psychology side (which is what most people think of when they hear psychology), and the BS side, which is oriented to the molecular side of things. A biochemistry minor is also useful. If you're really serious about this as a career, you should consider transferring to a school that offers a neuroscience program (not many do). I majored in neuroscience with a subtrack of psychobiology (@University of Miami), which only had 2 courses really relevant to psychopharmacology-- but it provides the background you need for studying psychoactives at the degree-earning level. Fortunately I was allowed to audit several graduate level courses to keep my interest going; but I wouldn't recommend this unless you're really that far ahead of the class on all the background subjects (undergrad neurobiology/psychobiology, genetics, cellular biology, organic chemistry, biochemistry, physics, calculus-- things you need to know before entering a psychopharmacology program). I plan on studying psychopharmacology in grad school next year as soon as I'm off house arrest (hooray for adjudication withheld). Pharacology/Pharmacy programs are more oriented towards drug development and molecular mechanisms of action, and far less concerned with psychoactives and cognitive effects.
But science majors are hard. Especially multidisciplinary ones that require upper level courses in the subjects I listed. Be sure you're really up for the kind of time commitment studying for all those things takes, and the years of background preparation before you get to the good stuff. I have to agree with The Is on this one; if you don't read science journals (and make that technical stuff like Nature Neuropsychopharmacology/Molecular Psychiatry; not simplified summaries of research like New Scientist) for fun, you should really have a hard look at whether this field is for you.
Edit: Check out ocw.mit.edu, the have online lecture notes; in the Health Sciences section there's lots of good material for neuroscience, pharmacology, and drug development.