Aspirin is compatible with phenacetin in mixtures but interacts slowly with paracetamol to produce increased amounts of salicylic acid and also diacetyl-p-aminophenol.The interaction is unaffected by caffeine but is accelerated by codeine phosphate and sharply by Mg stearate, with addnl. breakdown products including p-aminophenol being formed.p-Aminophenol is probably formed at a significant rate but is reacetylated by aspirin to paracetamol.Small amounts of diacetyl-p-aminophenol were found in a number of com. aspirin-paracetamol-codeine tablets.