Tetracycline is combined with which product to form doxycycline gel?

Doxycycline gel is formed by combining doxycycline with a biodegradable polymer. In this context, Atridox is the correct answer because it specifically refers to the doxycycline formulation that is delivered as a gel. Atridox contains doxycycline hyclate, which is used in the treatment of periodontitis by releasing the antibiotic over a period of time directly into periodontal sites.

Other products mentioned, like Arestin and Periostat, are also related to doxycycline but are used differently. Arestin is a minocycline microsphere, while Periostat is a systemic formulation of doxycycline used as an adjunct in the treatment of periodontal disease but not as a gel. Therefore, when looking for the specific formulation that utilizes doxycycline in gel form, Atridox stands out as the appropriate choice.