The RFU announced that the South African No8 will be suspended from all sports until September 2021 after the decision of an independent national anti-doping panel.
Three different steroids – Dehydrochloromethyl-Testosterone, Metandienone and Stanozolol – were found in his urine sample taken as part of an out-of-season test after a Carnegie training session in August last year.
Durban-born Staples – who made his debut for the club last season – had his contract terminated following the original anti-doping tribunal on November 30.
The 21-year-old also lost an appeal and a Carnegie statement read: “During the investigation process, Staples was offered support from the club.
“He has maintained that the failed test was as a result of health product he had purchased himself whilst at home in his native South Africa on holiday during the summer.”
In the statement, Hetherington added: “We are naturally very disappointed with the actions of Brandon Staples, who has let himself and everyone at the club down. The four-year suspension sends a stark warning to all professional sportspeople.”
Staples made just four appearances for Carnegie, making a try-scoring debut against a Dragons Premiership Select side in December 2016 before also featuring versus Bedford Blues, London Irish and Ealing last season.
The former Woodhouse Grove School student did not, however, play at all in the current campaign.
He represented Yorkshire Under-18s, attended Northumbria University and played at Darlington Mowden Park on dual-registration having originally moved to England in 2014.
RFU anti-doping and illicit drugs programme manager Stephen Watkins added: “This is the first failed test for a performance enhancing substance in English professional rugby since 2011 and a reminder to all that we cannot be complacent in our efforts to keep rugby as a clean sport.
“Education and deterrence remain core pillars to our anti-doping strategy.
“We must ensure that we continue to educate players of the risks involved in supplementation.
“Ultimately individuals are responsible for what they put into their bodies and this case highlights what can happen if a player is not vigilant.”