I agree with the insulin comments: nobody's going to care about that. I also agree that "boyfriend" is very often a red flag and is best avoided. But "friend" can also be a red flag (precisely because it's often viewed as a synonym for "boyfriend", especially when said by a young woman). My suggestion is to to answer the initial question with what you're planning to do in the US, not where you're staying. If you're going to be taking a trip someplace or similar sightseeing, say it. If all you're planning on doing is spending the 90 days at your boyfriend's place and hanging out, expect a grilling because those are exactly the people that CBP sees as "high risk". Your return ticket won't be worth much: what you need to show is that you have ties to the UK, such as a place you're paying rent on, a school you're enrolled in, a job, etc. If you can't show that you have something that will bring you back to the UK, there's indeed a real chance that you won't be admitted.

I'm a US citizen who's a male in my 50's with a professional, full-time job and an apartment that I've rented for decades, so very low risk. When I go to Canada, if my initial answer to what I'm doing there is "sightseeing and maybe meeting up with a friend", there's very little questioning (just the name of the hotel I'm staying at and what my job in the US is). But if I lead with "I'm visiting a friend", there are a series of questions about how long I know the friend, how I met them, etc.