Yesh. OK, I'll try to help, but still, yeesh
Hi. I am writing a novel and I'm interested in a few of things regarding formaldehyde.
Let me start off with my usual point. What you want is
verisimilitude -- a subtle bit of the appearance of knowledge, even with some falseness in it. That is more believable than perfect facts, which people will always try to pick apart.
Formaldehyde is toxic, and corrosive. We usually don't have to qualify it better than that. But anyway ...
-What exactly would happen if you injected formaldehyde into a human's blood stream?
Now see, you jump right in with two bad ideas at once. You know is toxic by ingestion, and of course if it burns skin if it touches, and irritates mucus membranes if inhaled, then you don't want to inject it. But you never want to inject anything that isn't an isotonic solution.
-What is the lethal dosage to kill an adult human?
LD
50 is 100 mg/kg (oral, rat). That is to say that 50% of rat subjects die at that dosage. Probably just as lethal to humans, so you just multiply as necessary.
-What would happen if you injected formaldehyde directly into the heart?
Bad things. just like I said above, but more so. For obvious reasons.
-Is using formaldehyde an efficient way of killing someone?
Ho-Kay. See, that's a bad question to ask. We're not in the business of planning murders for people. Even for literary sources. So you have to just work with the theoretical on your own.
-Can you provide some cool / off-beat facts regarding formaldehyde?
Try wikipedia. It often has random off-beat facts included. If they're not perfectly sourced, then that's verisimilitude -- a subtle falsehood that sounds good and is more believable than the truth.