Tetracyclines

When introduced, the tetracyclines were the first broad-spectrum antibiotics. They are able to block the transfer of RNA and prevent protein synthesis.

The tetracyclines are bacteriostatic for many Gram-negative and Gram-positive organisms. They inhibit the growth of mycobacteria, rickettsiae, mycoplasmas, psittacosis-trachoma group, and some protozoa.

The Tetracyclines vary only slightly with regard to their rates of absorption, distribution and excretion, with Doxycycline having the highest percentage of absorption. Because the tetracyclines are absorbed mostly in the small intestine, their absorption can be impaired by food, and dairy products. Tetracyclines all chelate with the metals, and therefore should not be given at the same time as aluminum, magnesium, or calcium containing antacids, or ferrous sulfate (example: multivitamins).

A notable exception to the above is that of Doxycycline which can be given with food or dairy since it is only about 20% hindered by their intake, and not deemed to be clinically significant.⁶

Tetracyclines will bond to calcium in newly forming bones so should not be administered during pregnancy or in the very young. Those with impaired renal function will require dose reductions; although some of the tetracyclines will vary in intensity and rate of secretions. However, here again, Doxycycline also differs in that its elimination is through non renal mechanisms and requires no dosage adjustment in renal impairment making it the drug of choice of the tetracyclines to be given.

Because Tetracyclines are so broad-spectrum they are very effective in the treatment of mixed infections.
This group of drugs is relatively nontoxic, however, they can modify the normal flora of the GI tract which can cause gastrointestinal disturbances. They may, also, if given in large doses produce liver damage.

Tetracyclines vary in their excretion from the body through feces or urine.

It is also important to note that decomposed tetracyclines (which includes doxycycline) may cause potentially fatal nephrotoxicity (Fanconi’s or Fanconi’s-like syndrome); therefore it is strongly recommended to discard any outdated (expired) or decomposed medications properly.