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One among the first antibiotics to come into use in human beings,
these drugs have stood the test of time and are continuing to be useful in treating a
broad range of infections, including malaria.
Mechanism of action: Tetracyclines are bacteriostatic agents,
supposedly acting by inhibiting protein synthesis by binding to the 30s ribosome subunit.
They are effective against a wide range of organisms, including aerobic and anaerobic gram
positive and gram negative bacteria, Rickettsia, Coxiella burnetii, Mycoplasma,
Ureaplasma, Chlamydia, Legionella, Spirochaetes, Brucella, Helicobacter pylori,
Yersinia, some atypical mycobacteria and Plasmodia.
Absorption, fate and excretion:
These drugs are incompletely absorbed form the gut after oral administration and the
absorption may be hampered by antacids containing aluminium hydroxide; calcium, magnesium
and zinc salts and bismuth subsalicylate. They distribute widely in the tissues and
accumulate in liver, spleen, bone marrow, bone, dentine and enamel of un-erupted teeth.
The drug is mainly excreted through the kidney 9except minocycline) and that may be
hampered in renal failure.
Adverse effects: Gastrointestinal
irritation, nausea, vomiting, diarrhoea, photosensitivity, hepato-toxicity, aggravation of
uremia, hypersensitivity reactions, staining of the teeth if used in young children and
pregnant women etc.
Use in malaria: These anti
microbials are useful in the treatment of drug resistant P. falciparum malaria.
They act relatively slowly and hence should always be combined with a faster acting drug
like quinine. They are contraindicated in children below the age of 8 years and in
pregnant women because of their adverse effects on bones and teeth.
Both tetracycline and doxycycline are
equally effective. Tetracycline is given at a dose of 250 mg every 6 hours for 7-10 days.
Dose of doxycycline is 100 mg twice daily for 7-10 days.
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