Penicillins should never be mixed in the same syringe with aminoglycosides because of their physical and chemical incompatibility. When ampicillin is combined with allopurinol, the risk of ampicillin rash increases dramatically. The administration of high doses of benzylpenicillin in combination with potassium preparations determines an increased risk of hyperkalemia. Extreme caution should be exercised when using penicillins active against Pseudomonas aeruginosa in combination with anticoagulants and antiplatelet agents due to the potential risk of increased bleeding. It is not recommended to combine penicillins with thrombolytics. Do not use penicillins in combination with drugs related to sulfonamides, as this reduces their bactericidal effect. Cholestyramine binds penicillins in the gastrointestinal tract and reduces their effectiveness when taken orally. Oral penicillins may reduce the effectiveness of oral contraceptives.

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