Introduction
Doxycycline is a member of the class of tetracycline antibiotics. These agents are effective against a wide variety of bacteria, including both Gram positive and Gram negative, specifically rickettseae, mycobacterium, and protozoa.


The basic structure of doxycycline contains a napthacene ring structure with a number of different groups attached at various sites. Tetracyclines as a class have problems with chelation with metals, and therefore should not be given with antacids, minerals, or milk. They are also somewhat unstable because of the formation of anhydrotetracycline in acidic medium, isotetracycline in basic medium, as well as an epimerization reaction in acidic medium, all of which lead to either inactive substances or nephrotoxicity in the case of anhydrotetracycline. The first two reactions mentioned both involve the 6-hydroxyl group. Doxycycline has significant improvements to the original tetracycline, including a replacement of the 6-hydroxyl group with a hydrogen, allowing for better stability in both acidic and basic conditions, as well a better absorption profile and a longer half-life.

Tetracyclines work by binding reversibly to the 30S subunit of the bacterial ribosome. This binding prevents the attachment of the amino acyl tRNA, which disrupts the translation process and prevents proteins from being synthesized. This class of drugs has a selective toxicity because they bind more readily to the bacterial 70S subunit of the ribosome compared to the mammalian 80S subunit. These drugs are transported efficiently into bacterial cells through the porins of Gram negative bacteria and through lipophilicity of Gram positive.

The mechanism of resistance of doxycycline, and tetracyclines in general, involves the production of bacterial proteins that, even with the drug bound to the ribosomes, allows the continuation of protein biosynthesis. At this time, it is not understood exactly how this happens. Another mechanism of resistance is through the active efflux of the magnesium-chelated drug to the outside of the bacterial cell in exchange for protons.

Clinical Use of Doxycycline

Doxycycline (also known under the brand names of Adoxa, Monodox, and Vibramycin) is used for a variety of indications, including anthrax, chlamydia, gonorrhea, Lyme disease, Rocky Mountain Spotted Fever, and malaria, among others (KB5). This drug is available in 50 or 100 mg capsules, 50, 75, or 100 mg tablets, and 25 mg/5 ml powder for oral suspension. Children under the age of 8 should not take this medication except for extreme situations, and it is not recommended for women in their last half of pregnancy. Doxycycline should not be taken with dairy foods, antacids, or minerals. It can however be taken with food to reduce the risk of gastric irritation. Side effects include photosensitivity, discoloration of teeth in young children, GI disturbances, and an increase in serum BUN. Patients should be advised to stay out of the sun or use protective measures while on this medication.


Though doxycycline contains significant improvements from earlier tetracyclines, because of widespread resistance and newer agents that do a better job, the future use of doxycycline will most likely continue to decrease. If additional improvements are made to combat the resistance, this class of drugs could continue to be effective antibacterial agents