Sean Stephenson, 18, had not been seen by his family since June 15 after he disappeared unexpectedly for a ‘trip to Canada’.
His family soon found out he had been arrested at Tokyo International Airport on June 21, allegedly in possession of a more than a kilo of methamphetamine.
His sister, Frederica, said “vulnerable” Sean was duped into smuggling the drugs by others.
She said Sean, who has never been in trouble with the law, must have been coerced into the act.
She said: “Sean is a kind, gentle, and trusting young man with autism and multiple physical and mental health challenges.
Japan has some of the strictest drugs laws in the world.
If convicted Sean is likely to face a lengthy custodial sentence.
A spokesperson for the Foreign Office said: “We are supporting a British man who is detained in Japan and are in contact with the local authorities”
