In Japan it is sometimes said a remedy for the common cold is to wear a warm green onion (Allii Fistulosi) around the neck. This practice is not as popular as before, but still many other articles about the topic can be found on the Internet.
Is there any study showing this is an effective cure?
"Green onion cures common cold": The old saying is actually true!
It is said that in green onion there is an allyl sulphuration component, which has an antibacterial/antimicrobial and bactericidal/germicidal action, and by wrapping it around the neck, the allyl sulphuration is absorbed via the mouth and nose, which sterilizes the nose and throat.
Another article focuses on simply alleviating the symptoms of common cold: http://www.houstonspresidentialsummit.com/cold/minkan01.php
Most people think that wrapping a green onion around the neck means that substance gets into the threat by contact, but in fact the most efficient way the substance enters the body is by gas from the wrapped green onion. The scent of the green onion has components that act as anti-inflammatory and sterilizer, allowing nerves to relax allowing better sleep.