Tetracyclines are broad-spectrum antibiotics. They may be
§ intermediate-acting compounds such as
§ long-acting compounds, such as doxycycline
hyclate and minocycline hydrochloride.
Tetracyclines are absorbed from the stomach and
small intestine when taken orally.
Tetracyclines are distributed widely into body
tissues and fluids, concentrated in bile, and excreted primarily by the
kidneys. Doxy-cycline is also excreted in stool. Minocycline undergoes
All tetracyclines are primarily bacteriostatic,
meaning they inhibit the growth or multiplication of bacteria. They penetrate
the bacte-rial cell by an energy-dependent process. Within the cell, they bind
primarily to a subunit of the ribosome, inhibiting the protein syn-thesis
needed to maintain the bacterial cell.
Tetracyclines provide a broad spectrum of activity
grampositive and gram-negative aerobic and anaerobic bacteria
The long-acting compounds doxycycline and
minocycline provide more action against various organisms than other
Tetracyclines are used to treat Rocky
Mountain spotted fever, Q fever, and Lyme disease. They’re the drugs of choice
for treating nongonococcal urethritis caused by Chlamydia and Ureaplasma.
Combination therapy with a tetracycline and
streptomycin is the most effective treatment for brucellosis.
Tetracyclines in low dosages effectively treat acne
because they can decrease the fatty acid content of sebum.
Tetracyclines can reduce the effectiveness of
hormonal contracep-tives, which may result in breakthrough bleeding or
ineffective contraception. The patient taking hormonal contraceptives should
use a reliable, secondary method of contraception. Tetracyclines may also
decrease the bactericidal action of penicillin.
§ Other interactions commonly affect the
ability of tetracyclines to move through the body.
§ Aluminum, calcium, and magnesium antacids
reduce the ab-sorption of oral tetracyclines.
§ Iron salts, bismuth subsalicylate, and zinc
sulfate reduce the ab-sorption of tetracyclines. Reduced absorption can be
prevented by separating doses of tetracyclines and these agents by 2 to 3
§ Barbiturates, carbamazepine, and phenytoin
increase the me-tabolism and reduce the antibiotic effect of doxycycline.
These drugs, with the exception of
doxycycline and minocycline, may also interact with milk and milk products,
which bind with the drugs and prevent their absorption. To prevent decreased
ab-sorption, administer the tetracycline 1 hour before or 2 hours after meals.
(See Adverse reactions to tetracyclines.)