I think a lot of confusion surrounding minoxidil on this board is from people not realizing that the safe dosages of oral and topical minoxidil are very, very different. When used topically, minoxidil has a very low systemic absorption (which is part of the reason why it is a very good medicine to use typically) and only a tiny fraction will go systemic. The standard topical dose for hair loss is 100mg/day, the standard oral dose for hairloss is only 1-2mg per day, and the standard oral dose for hypertension is 1mg per day. You can see the difference! But it's also relatively safe to use even higher doses topically. Personally, I've used over 300mg per day topically. If I took that dosage orally, it'd kill me!
Something to keep in mind though is that when using topical minoxidil, some small amount will go systemic. It's hard to gauge how much that is, but I could be getting the equivalent of a few mg dose in my bloodstream just from the topical administration. Because of this you should use caution when using high topical doses if you have a heart condition and you should use caution if you're going to take oral minoxidil while using large doses of topical. You might think that 5mg oral isn't a lot, but it could be a lot if you combine it with lots of topical application