How does Codeine cause constipation

Codeine, a closely related substance to morphine (so codeine is also an opioid) converted in the body to morphine. Codeine is readily absorbed by mouth and it does not have the dependent effect of morphine. But since it is converted in the body to morphine it can cause various gastro-intestinal symptoms. So codeine increases the tone and reduces the motility of many parts of the gastro intestinal tract. This can delay the gastric emptying and considerably reduces the absorption of the other drugs. It can also increase the pressure in the biliary system since it increases the contraction of the gallbladder and increase the tone of the biliary sphincter. So codeine should be avoided in patients with biliary colic caused by gallstones. This action on gastro intestinal smooth muscles can cause severe constipation in some patients and it can be very troublesome to them.