aspirin (acetylsalicylic acid)

introduction

aspirin has analgesic, antipyretic, anti-inflammatory activity (similar to non-steroidal anti-inflammatory drugs (NSAIDs)), as well as prolonged anti-platelet activity
hypersensitivity reactions, particularly bronchospasm, may occur, particularly in those with asthma.
being a weak acid, it is more rapidly absorbed once past the acidic environment of the stomach, and thus onset of actions is hastened if taken on an empty stomach, with caffeine, or with a promotility agent which hastens gastric emptying such as metoclopramide (Maxolon)
aspirin, as with non-steroidal anti-inflammatory drugs (NSAIDs) may cause gastritis or gastric ulceration with or without occult blood loss, particularly when taken with alcohol or with nsaids
aspirin is strongly protein bound and may displace other drugs from protein binding sites resulting in increased and prolonged pharmacological effect of those drugs eg. phenytoin, sodium valproate (Epilim), sulphonamides, and methotrexate.
high dose aspirin may enhance the hypoglycaemic effects of sulphonylurea agents and reduce the activity of probenecid and other uricosuric agents
it may antagonise the diuretic effects of spironolactone
long term low dose aspirin in those over 45yrs of age appears to be associated with double the risk of wet macular degeneration (5.8% vs 2.3% after 15yrs in one small study) but reduces the risk of AMI, stroke and possibly, some forms of cancer.

It inhibits platelets by irreversibly covalently acetylating the active site of cyclooxygenase which is the enzyme that produces the cyclic endoperoxide precursor of thromboxane A2.
Since platelets do not synthesise new proteins, this action is permanent, lasting the life of the platelet (7-10days).
Repeated doses produce a cumulative effect on platelet function.
Complete inactivation is achieved when 160mg is taken daily.
300mg is given to be chewed in a single dose as early as possible in pts with AMI.
Higher doses do not improve efficacy but increase bleeding & produce aspirin's other actions.

usual formulations are 300mg (eg. Disprin) although some stronger formulations have 500mg (eg. Disprin max)
some formulations also have codeine in 8-15mg per tablet (eg. Aspalgin)
usual adult dose 300-600mg qid
usual maximum dose is 1000mg qid