Tetracyclines have the broadest antimicrobial spectrum.They intefere with bacterial protein synthesis.They are bacteriostatic and resistance to them develops frequently.They are active against many gram positive and gram negative pathogenic bacteria strains except for those of Pseudomona aeruginosa.
Tetracyclines are best used for minor infective conditions in penicillin allergic oral and antral cases.It is only partially absorbed from the gut,sufficient concentrations remain in the intestine to alter the flora which may give rise to local complications such as discomfort,diarrhoea and suppression of vitamin K synthesis.Other problems include heartburn,nausea and vomiting due to gastric mucosal irritation,skin rashes and photosensitization.
Absorption is reduced by chelation to calcium in dairy products and even more so with antacids and iron preparations.The plasma half life varies with the tetracycline and is increased in renal failure except for doxycycline which is not eliminated by the kidney and therefore may be used with impaired renal function.
Superinfection may occur with candida albicans producing a sore mouth or thrush in addition to a black hairy tongue,diarrhoea and pruritis.Superinfection with staphylococci,proteus or pseudomonas causing a fulminating fatal enteritis is also possible if used in heavy prolonged dosage.Oral,anal and vaginal superinfection can be avoided by use of nystatin or amphotericin and vaginal suppositories.Tetracycline are selectively chelated by the calcium of growing bones and teeth both in the foetus and children causing yellowish or brownish discoloration of the enamel.It can also cause enamel hypoplasia.Tetracycline must be avoided in children until twelve years of age.
Dosage is teracycline dosage is either capsules or tablets 250 mg 6 hourly.Demeclocycline is more readily absorbed from the gut and excreted more slowly and therefore 300 mg may be given 12 hourly.
Parenteral adminstration is best by intermittent intravenous infusion of 1 gram of tetracycline or oxytetracycline in 0-9 percent sodium chloride each 24 hours divided in to 2 or 4 doses each infused over 5 to 10 minutes.Intramuscular injections are painful.