also have activity against certain eukaryotic parasites, including those responsible for diseases such as dysentery caused by an amoeba, malaria (a plasmodium), and balantidiasis (a ciliate). Use as a biomarker Since tetracycline is absorbed into bone, it is used as a marker of bone growth for biopsies in humans. Tetracycline labeling is used to determine the amount of bone growth within a certain period of time, usually a period around 21 days. Tetracycline is incorporated into mineralizing bone and can be detected by its fluorescence. In "double tetracycline labeling", a second dose is given 11–14 days after the first dose, and the amount of bone formed during that interval can be calculated by measuring the distance between the two fluorescent labels. Tetracycline is also used as a biomarker in wildlife to detect consumption of medicine- or vaccine-containing baits. Side effects Use of tetracycline antibiotics can: Discolor permanent teeth (yellow-gray-brown), from prenatal period through childhood and adulthood. Children receiving long- or short-term therapy with a tetracycline or glycylcycline may develop permanent brown discoloration of the teeth. Be inactivated by calcium ions, so are not to be taken with milk, yogurt, and other dairy products Be inactivated by aluminium, iron, and zinc ions, not to be taken at the same time as indigestion remedies (some common antacids and over-the-counter heartburn medicines) Cause skin photosensitivity, so exposure to the sun or intense light is not recommended Cause drug-induced lupus, and hepatitis Cause microvesicular fatty liver Cause tinnitus Interfere with methotrexate by displacing it from the various protein-binding sites Cause breathing complications, as well as anaphylactic shock, in some individuals Affect bone growth of the fetus, so should be avoided during pregnancy Fanconi syndrome may result from ingesting expired tetracyclines. Caution should be exercised in long-term use when breastfeeding. Short-term use is safe; bioavailability in milk is low to nil. According to the U.S. Food and Drug Administration (FDA), cases of Stevens–Johnson syndrome, toxic epidermal necrolysis, and erythema multiforme associated with doxycycline use have been reported, but a causative role has not been established. Pharmacology Mechanism of action Tetracycline inhibits protein synthesis by blocking the attachment of charged tRNA at the P site peptide chain. Tetracycline blocks the A-site so that a hydrogen bond is not formed between the amino acids. Tetracycline binds to the 30S and 50S subunit of microbial ribosomes. Thus, it prevents the formation of a peptide chain. The action is