Caffeine relaxes smooth muscle of the biliary and gastrointestinal tracts and has a weak effect on peristalsis. However, high doses can produce biphasic responses, with an initial contraction followed by relaxation. Caffeine seems to have no effect on the lower oesophageal sphincter. The increase in both gastric and pepsin secretions is linearly related to the plasma levels obtained after the administration of a dose of 4-8mgkg~1. In the small intestine, caffeine modifies the fluid exchange from a net absorption to a net excretion of water and sodium.
The role of caffeine in the pathogenesis of peptic ulcer and gastrointestinal complaints remains unclear, and no association has been found in clinical and epidemiological studies.
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