Why do people instinctively suck on a bleeding wound?

We put Dennis' question to Dr Chris Smith...

Chris - It does sound counter-intuitive, doesn't it?

Probably, there are two aspects to this. Number one is that whenever you injure any part of yourself, the reaction or the reflex response is usually to rub it better. When you rub something better, what you're doing is stimulating the nerve cells that convey low threshold stroking sensations to your brain. Those nerve cells are wired up in your spinal cord to inhibit, or switch off, the nerve cells that convey pain. So, by sucking on an injured body part, you're stimulating those big low threshold nerves with that stroking sensation and thereby, blocking the pain.

Number two is that, actually, although your mouth is full of germs, they are a fairly unique repertoire of germs that include streptococci that we call Viridans streptococci, and some staphylococci; they're generally low pathogenicity bugs. They probably are not going to be bad for you in a big way. As a result, your immune system is pretty good at dealing with them, and if you pushed a few of them into your wound, they're probably not going to do much harm. Also, when you clean the wound with your tongue or by sucking, you're going to dislodge foreign matter. You're probably going to dislodge some nastier bugs that might be in there and they'll get swallowed and then destroyed by your stomach acid.

But here's the real sort of clincher for this argument, which is that, every time you clean your teeth, if you do tests on people you can detect showers of microorganisms - and their DNA - going around your bloodstream. Because, when you clean your teeth, you make little lacerations in your gums. Also, when you eat food, same story, when you floss your teeth, little holes open up in your gums and the bugs in your mouth get into your bloodstream.

In the vast majority of cases, they're harmless to you because your immune system deals with them straight away; your spleen filters them out, without a problem. In a small minority of people, usually people who've got pre-existing problems with the valves in their hearts, then these microorganisms can sometimes settle on the valves of your heart and they cause a condition called endocarditis. But for the vast majority of people, it is not a problem.

So, there's the evidence that you're probably not carrying anything in your mouth that you're going to add to your wound and make yourself sick with it...