"Because tetracycline can cross the placenta, it may stain primary teeth if taken during pregnancy," says the book Oral Pathology: Clinical Pathologic Correlations by Regezi. "If it is administered (to children) between birth and age six or seven years, permanent teeth may be affected."
Tetracycline is also one of the many antibiotic drugs commonly given to conventional livestock and poultry to make them grow faster. According to The Atlantic, the U.S. Food and Drug Administration(FDA) continues to allow the use of tetracycline in commercial agriculture, even though its persistent use is leading to the rapid emergence of antibiotic-resistant superbugs.(http://www.theatlantic.com)