Long-acting oxytetracycline composition

A composition containing as active principle an amount of a tetracycline compound effective for antibiotic activity, either as the free base or a salt thereof with a physiologically acceptable acid, complexed with a substantially equimolar amount of a magnesium compound, solubilized in a water miscible solvent system comprising,

• a) glycerol formal in an amount of from about 10 to about 50% (v/v);
• with
• c) polyethylene glycol wherein,
• (i) when the polyethylene glycol hag a molecular weight of not more than about 200, the polyethylene glycol is present in an amount of from about 1% to about 2(0% (v/v);
• (ii) when the polyethylene glycol has a molecular weight of not more than about 400, the polyethylene glycol is present in an amount of from about 1% to about 15% (v/v);
• (iii) when the polyethylene glycol has about molecular weight of not more than about 6000, the polyethylene glycol is present in an amount of from about 1% to about 10% (v/v);
• and
• (iv) when the polyethylene glycol has a molecular weight of greater than about 6000, the polyethylene glycol is present in an amount of from about 1% to about 5% (v/v),
• said composition optionally containing a pH modifier in an amount sufficient to maintain a physiologically acceptable pH, the balance being made up with water q.s.