Stomach, normal flora

Exterior defenses Skin, mucus, cilia, normal flora, saliva, low pH of the stomach, skin, and genitourinary tract None... 

These antibiotics are partially absorbed from the stomach and upper gastrointestinal tract. Food impairs absorption of all tetracyclines except doxycycline and minocycline. Absorption of doxycycline and minocy-cbne is improved with food. Since the tetracyclines form insoluble chelates with calcium (such as are found in many antacids), magnesium, and other metal ions, their simultaneous administration with milk (calcium), magnesium hydroxide, aluminum hydroxide, or iron will interfere with absorption. Because some of the tetracyclines are not completely absorbed, any drug remaining in the intestine may inhibit sensitive intestinal microorganisms and alter the normal intestinal flora. 

For perhaps a last word, it is said that amygdahn or laetrile will release cyanide in the stomach by the action of the enzyme P glucosidase, also called emulsin (Smith, in Amdiu et al., 1991, p. 277). Not found in mammalian tissues, such enzymes occur in normal human intestinal flora, or gut flora. For this reason, amygdalin is about 40 times more toxic by mouth than by intravenous injection. Thus amygdalin or laetrile could be given safely by parenteral routes. (The term parenteral signifies routes other than oral administration.)... 

---