I am currently doing pharmacology. Most of the content is related to neuroscience e.g. how drugs act on the receptors to create a desired effect, pharmacokinetics, and physiology.
We also have to take certain biochem modules, biochem seems to be more related to protein structures, genetics, and perhaps metabolism in the body, which is also the foundation for pharmacology.
I wasn't sure if I should choose pharmacology or biochem in my high school as well. I think it might be helpful to take a look at the module information for phar and bioc in university official page and perhaps their possible careers, and decide which one u like more. Hope it helps.