how "dangerous" is it to double-dose medication?

You're operating on the assumption that more is better --- it isn't. With all medications, there's a zone known as the "therapeutic index" -- in essence, it's the range of doses where the benefits of the medication outweigh the risks. Below this range, you don't get benefit, but you're unlikely to experience bad side effects. Above this range, the risk of serious toxicity increases, while you don't get much increase (if any) in how effective the medication will be. Most nonprescription medications have a pretty wide therapeutic index, meaning that as long as you follow the instructions on the bottle, the doses will be safe for the vast majority of the public. However, when you increase the doses on your own, you can run significant risk of toxicity, especially with products containing acetaminophen. While you sarcastically ridicule your friend's 6 months of training, I'd point out that this is six month of formal training you haven't had..... Your friend is absolutely correct in that you should not be doubling the doses on your own -- you're not really getting extra benefit, but you are getting extra risk....