Dear Rabbi Simon
I know that it is forbidden to eat human blood. I realized that I had a cut on my thumb while flossing and it was actually bloody. I don’t think that it actually got in my mouth but it may have touched my lip. I did not swallow it. Is this a problem?
You are correct that it is forbidden to consume human blood (including your own). However this is a rabbinic, rather than a Torah prohibition, based on the principle of marit ‘ayin (that is, the appearance of impropriety), since human blood can be confused with the blood of an animal, which is forbidden by Torah Law. For this reason one whose gums are bleeding, for example, is not required to go to lengths to avoid ingesting blood, since the concern of marit ‘ayin is inapplicable. On the other hand, if one becomes aware of a wound in his mouth through the presence of blood on a piece of bread (for example) s/he must remove the surface of the bread where the blood is found. The rest of the slice of bread may be consumed.
With this in mind, in the case you describe, if you rinsed your mouth and avoided swallowing the blood from the cut on your thumb, there is no problem.
I hope that by now your thumb is healed.
Rabbi Rashi Simon