Weak opiates
Codeine and tramadol are used for
moderate to severe pain and are
pro-drugs for morphine (designed
to improve bioavailability when
a drug itself is poorly absorbed).
Once absorbed they are converted
(between 10–20%) into the active
metabolite associated with morphine
in the liver. For example, 60mgs
codeine is approximately equivalent
to 6–12mgs morphine. However,
10% of people do not have the
enzyme to convert weak opiates and
do not find them effective (Bond and
Simpson, 2006). Tramadol has less
of an effect on respiratory function
and a lower effect on gut motility,
resulting in less constipation
compared to codeine (Bond and
Simpson, 2006).