They told a meeting of the American Chemical Society in Las Vegas that about
one in 10 whites and two percent of Chinese lack an enzyme needed to process
the drug.

Their findings should be taken in account by doctors prescribing such drugs,
they said.

Codeine is usually converted into morphine by the body. But Alastair Wood and
colleagues at Vanderbilt University in Nashville found some people cannot do
this.

They lack the enzyme CYP2D6, which also breaks down drugs including
propanolol, used to treat high blood pressure, propafenone, for heart
arrythmias and some tricyclic antidepressants.

But Wood said this meant such drugs had an exaggerated effect on those people
because they act directly on the body rather than through a metabolite, or
breakdown product.

``Most drugs themselves produce the effect so if a patient doesn't get the
effect, the physician gives more of the drug,'' Wood said in a statement
released by the American Chemical Society.

Codeine was different. ``Here's a situation in which a proportion gets no
effect. It's actually predictable.''

Wood's team has been studying different effects of morphinebased drugs on
different ethnic groups. They said many previous studies had found that
racial differences do affect how people respond to drugs from alcohol to
betablockers used to treat high blood pressure.

``In spite of the recognized interethnic differences in drug disposition and
sensitivity, little attention has been given to ethnicity as a factor in
determining drug dosage in different populations,'' they said in an earlier
report on the different ways people of Chinese and European origin react to
morphine.

They said prescription recommendations often assumed people around the world
would react in the same way to the same drugs, but their studies showed this
was untrue.