When your mathematics is ready for it. Also, many medical schools will not accept AP/IB credits for required prerequisites, so if you take it, you need to have dual enrollment or some university grades.am currently in high school, but am able to take college level courses and have taken many before. I am thinking about majoring in biochemistry and would need a calculus-based physics class as a requirement (I have taken calculus I and II before and did fine at the college). I also have the option of taking it at the high school, but I've heard it is not taught that well. However, if I take it at the college level and get credit for it as a senior in high school, I'm worried that it I will forgot some of the material by the time I take the MCAT. Of course, I recognize that plans might change, but I do enjoy a challenge and learning about science, I'm mostly concerned about forgetting it before the MCAT since that's pretty much a 3 year time gap.
(I'm also worried that it will be very hard. I breezed through calculus I and struggled a lot in calculus II, but still did fine in the end. How hard is calculus based physics compared to calculus II?)
Really to say on difficulty. I find most people fail calculus because their trig knowledge is deficient in the first term, but the second when you start talking about series and partials are the first real issues in calculus where you can mess up by the concepts alone rather than a mechanical ability issue. For physics (as a non-physics major), I felt that the lack of math skills was what doomed more students than the actual subject matter. That said, you do need to think somewhat creatively in problem formation in physics more so than you have done in the past sciences. Physics problems lend themselves to too much information and hard to set up solution scenarios more so than Chemistry (until you learn mechanisms in Organic Chemistry).
If you have not taken AP Chemistry, you should do so even though you are going to repeat it in university studies. You want to have a good foundation walking into university with that as the freshman sections can be brutal due to the diversity of background knowledge brought in with a competitive curve scoring dynamic.
If you are majoring in Biochemistry, then you really ought to have AP Chemistry and preferably also some AP Biology (the old Campbell book that they tend to use is very decent at teaching fundamental Biochemistry that you will see again in a harder fashion later).