The prohibition against ingesting blood is a very severe one.[1] However, the full force of the prohibition does not apply to human blood.[2] In fact, according to most opinions, ingesting human blood is forbidden only mid’rabbanan (rabbinically).[3] Furthermore, the rabbinic prohibition is not a blanket rule that treats human blood as inherently prohibited. Rather, the prohibition depends on the circumstances, as we shall see.
The gemara[4] starts by discussing whether people are permitted to drink mother’s milk and brings an apparent contradiction on the matter. The gemara concludes that it is permitted to drink human milk after it has been removed from the body, but not directly from a mother, which is permitted only for babies. It then points out that the opposite is true for human blood, as illustrated by a baraita. The baraita says that one may swallow blood that is found in between his teeth, but not if it has found its way onto the piece of bread he is eating. Rashi explains that once it is on the bread, someone might think he is eating animal blood that got on the bread. Following this approach, the Shulchan Aruch[5] calls this a prohibition of marit ayin (when the action that people think they are seeing is a forbidden one). In the case of the blood on the bread, it appears that he is ingesting animal blood.
The question then becomes: which situations are included in the marit ayin prohibition? Tosafot[6] says that one is permitted to lick the blood that drips from a wound on his finger because it is clear, from the fact that the blood is taken directly from the body, that it is human blood. However, the Minchat Yaakov, in his commentary on the Torat Chatat,[7] infers from Rashi in Ketubot[8] that he disagrees. Rashi says that the reason it is permitted to swallow blood from between the teeth is that “there is none who sees it.” The Minchat Yaakov reasons that according to this approach, it is forbidden to suck blood from the wound on a finger because it can be readily seen. The Darchei Teshuva[9] cites additional opinions on either side.
What about our case, regarding blood coming from cracked lips? According to Tosafot, it is clearly not a problem for him to swallow the blood. What about according to Rashi? One could claim that if the blood is visible, then it is a problem. However, it is more likely that when Rashi wrote that “there is none who sees it,” he was referring not to the blood itself but to the ingestion of the blood. While one can notice a person licking the blood off his finger, one cannot usually notice when some blood is licked from the lip into the mouth.
One can make the same calculation with regard to the use of dental floss. (I know there are people who would enjoy a halachic excuse not to floss, but they will have to go elsewhere to find one.) It is true that the floss sometimes picks up some blood, which is noticeable, and that it is not always cleaned before being put back into the mouth, where some may be ingested. According to Tosafot, it is clear that blood found on dental floss comes from a human, and therefore presents no problem. Even according to Rashi, it is not clear to one who sees the flossing that any of the blood will be ingested, and the ingestion will not be visible. Therefore, it is likely that both would agree that it is not necessary to clean the bloodstained dental floss before reinserting it into the mouth. Furthermore, the Kaf HaChayim[10] says that there are more who agree with Tosafot than with Rashi, and one can rely on the lenient opinions in this rabbinic matter.