About Dexamethasone A potent synthetic glucocorticoid, Antiasthmatic, anti-inflammatory and immunosuppressant. Interactions for Dexamethasone ENDOCRINE SYSTEM: Barbiturates: Decreases efficacy of dexamethasone. Carbamazepine, Primidone: Decreases efficacy. Oral contraceptives: Increases efficacy of dexamethasone. Ephedrine: Decrease efficacy of dexamethasone. Oestrogens: Decrease efficacy of dexamethasone Hydantoins: Decrease efficacy of dexamethasone Ketoconazole: Increase efficacy of dexamethasone Rifampicin: Decreases efficacy. Dexamethasone effects the actions of the following: Anticholinesterases: Efficacy antagonised in myasthenia gravis. Oral anticoagulants: Altered response. Cyclosporine: Increased cyclosporine efficacy leading to enhanced toxicity. Digitalis glycosides: Increased toxicity associated with hypokalaemia. Isoniazid: Decreased serum levels of isoniazid. Salicylates: Decreased serum levels of salicylates. Diuretics: Increase efficacy