The speedy winger was at the peak of his powers when he travelled to Argentina for the 1978 World Cup as part of the Scotland team. He was also a hay fever sufferer, and after a farcical 3-1 defeat against Peru he was tested and found to have taken Fencamfamin (a sympathomimetic amine with structural and biochemical similarities to amphetamine and cocaine, of course).

He protested his innocence, citing hay fever potions and a mysterious "flu-tonic", and Scotland's World Cup campaign crumbled. Johnston was sent home, banned from international football and briefly emigrated to the United States, with only a huge cheque from a tabloid newspaper to comfort him in his disgrace.