The answer lies in how codeine works relative to others in this drug class.

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But if you are and take codeine, you convert much more of it to morphine than in most people.

FORBES: Why Codeine is Dangerous in Some Kids After Tonsillectomy

Doctors say the cough syrup contains codeine phosphate which, if taken beyond prescribed limits, makes people "drowsy and intoxicated".

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It turns out that those who can't turn codeine into morphine lack an enzyme that is controlled by their genes.

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Most of the painkilling effects of codeine are due to some of it being converted to the strong opioid analgesic, morphine.

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However, a small percentage of people are born with a single-letter change in their DNA for this enzyme that causes the codeine-to-morphine reaction go faster.

FORBES: Why Codeine is Dangerous in Some Kids After Tonsillectomy

Scientists have found that the genetic mutation which renders codeine ineffective is much more common among northern Europeans, than those who hail from the Mediterranean.

BBC: Scientists hope to develop individually-targeted drugs

Dr Sharma said the new seed has no trace of any of the natural chemicals which cause a narcotic effect - namely, morphine, codeine and narcotine.

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The other 90% of the population who take the medicine are able to break codeine down in their cells to make morphine and hence receive a significant amount of pain relief.

BBC: Scientists hope to develop individually-targeted drugs

But it would be as wrong to conclude that giving codeine to somebody from Malta would be bound to work, as it would to assume that giving it to a Swede would be a waste of time.

BBC: Scientists hope to develop individually-targeted drugs