Niteworks is made from L-arginine, an amino acid that helps produce nitric oxide.

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The study showed that by substituting salmeterol for an alternative anti-inflammatory medicine, montelukast, outcomes for severely asthmatic children with the arginine-16 gene variant were substantially improved.

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John's wort, the authors of the review found "good but limited" evidence for the anxiety-reducing effects of supplements containing L-lysine and L-arginine, two amino acids produced naturally by the body.

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The drug, is found in Seretide and Servant inhalers, acts on beta-2 receptors in the lining of the airways, but previous research has shown that as many as one-in-seven children carry a gene variant - arginine-16 - that undermines its effectiveness.

BBC: Is this the shape of medicine to come?