Figure
7.2 Laxative And Opioid Doses
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| A
graph to show the relationship between opioid and laxative
doses. Mean laxative doses are displayed by quartile
of mean opioid dose. (Note that non-opioid takers are
shown for comparison and 95% confidence intervals for
the means are indicated.) (Sykes,
1998) |
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An
intrinsic difficulty in this type of study is that of deciding
a basis for the comparison of different laxatives. It is rarely
practicable in a study of any size in a general clinical setting
to completely control the types of laxatives that are given, principally
because of issues of patient tolerance. Also, it is probably appropriate
in opioid-induced constipation that a combination of softening
and stimulant agents is used. How are these different drugs to
be combined into an expression of "laxative dose"?
The
simplest method is to ignore the fact that different preparations
are involved and to calculate the volume or number of tablets
that are given. If a mixture of tablets and liquids is used some
judgment needs to be made of their relative potency. This is easy
if the same drug is involved but less straightforward if they
are not, because there are few data by which to compare laxative
potencies.
Different
investigators utilize different methods for standardizing or quantifying
laxative potencies.
Currently,
the accuracy of any system of comparing laxatives cannot be known
clearly. In a comparative trial the level of distortion will depend
on the degree to which the groups received an equal variety of
laxative preparations. In a trial such as that of the constipating
effect of morphine the assumption is that no particular laxative
was used preferentially within one morphine dose range more than
others.
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