Okay, since this has come up quite often, I thought I would do some research on it. So here's what I read. The custom is ancient but not in the Bible. It's an old tradition.
Apparently the roots of the tradition have several aspects:
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In ancient times, it was considered a quick way to stanch the flow of blood, and it was believed to be medically necessary.
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The saliva was considered to have antiseptic properties, and to be a hygienic measure.
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The custom of circumcision included that some blood should flow, and the suction ensured that a little bit would, making the ritual effective. The drop of blood was an essential part of a sacred ritual.
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The one performing the circumcision did not remove the foreskin with his mouth. This was done with a knife or surgical instrument. The suction was only performed after the circumcision as an attempt to speed healing.
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In recent times, with the increase in sexually transmitted diseases, the custom has been replaced by use of a sterile tube although some very traditional communities still practice it.
https:// en.wikipedia.org/wiki/Wound_licking
http:// www. jewishanswers.org/?p=790
https:// en.wikipedia.org/wiki/Brit_milah
http:// www.tabletmag.com/jewish-life-and-religion/124412/birth-right-3