Johnson confident England are safe from drug trouble

Springboks Bjorn Basson and Chiliboy Ralepelle were sent home from the tour after testing positive for the banned stimulant methylhexaneamine after South Africa's victory over Ireland.

As a direct consequence, UK Sport tested six South Africa players following their defeat to Scotland on Saturday night instead of the usual two.

The Springboks have stopped the use of all supplements and are trying to investigate the source of the positive tests, with team doctor Craig Roberts describing the whole area as "a minefield".

Methylhexaneamine was originally intended to be used as a nasal decongestant but in recent times it has found its way into dietary supplements. It normally stays in the body for 24 hours.

Johnson admitted he was surprised rugby has not been hit by more incidents like this because of the dangers of contamination – but he is confident England's set-up is watertight.

"We haven't changed anything about what we do because we are careful anyway," said Johnson.

"You have got to be very careful. Tiny amounts of substance can get detected nowadays and suddenly you fail a drugs test. It is very easy for that to happen.

"Even when I was playing, way back in the distant past, there was fear of cross-contamination. It is amazing there isn't more.

"We had the testers in last week. There has been a lot of testing going on and I am very happy that what we have got in place with our guys is thorough. It has to be."

The RFU recently passed on an International Rugby Board warning to member clubs about methylhexaneamine, advising players to be "extra vigilant when using dietary or nutritional supplements".

The England squad – like every team ranked in the IRB's top 20 – are subject to the World Anti-Doping Agency's player whereabouts scheme.

"Our guys have to be where they say they'll be for an hour a day for the drugs testers," said Johnson.

"They can get tested in their allotted hour, they can get tested randomly. They can get tested after matches and they know that."

South Africa centre Jean de Villiers insisted he did not use supplements anyway – but he denied last week's upheavel was to blame for their 21-17 defeat to Scotland.

"It wasn't an easy week for us losing the two guys. A lot went wrong last week," he said.

"Losing to Scotland was a real low point. It was one of the worst moments I have experienced."