The concentration of oxytetracycline in plasma was studied by microbiological assay after oral administration of four different preparations of oxytetracycline dihydrate tablets. There were statistically significant differences in biological availability between the four preparations, as assessed by the peak plasma level, the area under the plasma concentration‐time curve, or the cumulative fraction of the dose excreted in urine at 405 minutes. In contrast, differences between the subjects were not statistically significant. The differences in biological availability were not predictably related to the in vitro dissolution of the tablets.

1 The concentration of oxytetracycline in plasma was studied by microbiological assay after oral administration of five different preparations of the antibiotic. None of these preparations had been studied previously. 2 There was a statistically significant correlation between the time required for 50% dissolution at pH 2 and biological availability, as assessed by the peak plasma level or the area under the plasma concentration‐time curve. 3 The mean bioavailability of oxytetracycline was greatest with preparations of the hydrochloride, and with film‐coated tablets of the dihydrate. In contrast, sugar‐ coated tablets of oxytetracycline dihydrate were associated with poorer dissolution characteristics and reduced biological availability.

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