It is thought to act through selective inhibition of central cyclo-oxygenase. While it has little gastric effect, damage due to overdose is irreversible and care must be taken to avoid polypharmacy problems. Codeine and dihydrocodeine act centrally on the opioid receptors. Although the dose of codeine in OTC preparations maybe at sub-therapeutic levels, its benefits may be synergistic and there is no doubt that patients frequently prefer combination analgesic products. The major side action of these drugs is constipation.
Caffeine acts as a stimulant but at the dose employed it probably has little effect.