PABA MAY INCREASE SERUM LEVELS. URINARY ALKALISERS, ANTACIDS, CORTICOSTEROIDS MAY INCREASE EXCRETION. CONVERSLY ASPIRIN TOXICITY MAY OCCUR WHEN STEROIDS ARE WITHDRAWN. MAY POTENTIATE THE EFFECTS OF ANTICOAGULANTS, HYPOGLYCAEMICS, METHOTREXATE, PHENYTOIN, VALPROIC ACID, TRICYCLIC ANTIDEPRESSANTS & ZAFIRLUKAST. MAY ANTAGONISE URICOSURICS LIKE PROBENECID & SULPHINPYRAZONE, SPIRONOLACTONE AND TETRACYCLINES. SALICYLATES INHIBIT THE METABOLISM OF VALPROIC ACID AS WELL AS DISPLACE IT FROM ITS PROTEIN BINDING SITE. FURTHERMORE ASPIRIN IS ASSOCIATED WITH INCREASED RISK OF REYE'S SYNDROME. VALPROIC ACID ANS ASPIRIN BOTH ARE HEPATOTOXIC AND BOTH HAVE EFFECT ON BLOOD COAGULATION & PLATELET FUNCTION. HENCE THEY SHOULD NEVER BE USED TOGATHER. SHORTENS THE TIME TO INDUCE ANAESTHESIA WITH MIDAZOLAM. GRISEOFULVIN REDUCES ASPIRIN ABSORPTION. THE ANTIPLATELET EFFECTS OF ASPIRIN AND CALCIUM CHANNEL BLOCKERS IS POTENTIATED WHEN BOTH ARE USED TOGATHER. INCREASED CHANCES OF BLEEDING, ECHYMOSIS OR BRUSING IS SEEN. CARBONIC ANHYDRASE INHIBITORS LIKE ACETAZOLAMIDE, METOPROLOL & METOCLOPRAMIDE INCREASE ITS ABSORPTION & PLASMA CONCENTRATION. GOLD COMPOUNDS MAY EXACERBATE ASPIRIN INDUCED LIVER DAMAGE. CONCOMITANT USE OF OTHER NSAIDS SHOULD BE AVOIDED BECAUSE OF INCREASED CHANCES OF SIDE EFFECTS. ASPIRIN REDUCES OR ALMOST ANTAGONISES THE HYPOTENSIVE ACTION OF ACE INHIBITORS DUE TO EFFECT ON RENAL PROSTAGLANDINS. INCREASED RISK OF HAEMOTOXICITY WITH ZIDOVUDINE, BLOOD COUNT AFTER 2-3 WEEKS OF STARTING TREATMENT IS RECOMMENDED. RITONAVIR MAY INCREASE THE PLASMA CONC OF NSAIDS.