thgWhat is THG?
THG is short for Tetrahydrogestrinone and is a drug that helps people to run quicker. It's an anabolic steroid.
Anabolic steroids make it easier to train and compete. Athletes aren't as tired after they train, and recover much more quickly. They also make it easier for people who use it to build bigger muscles, which helps them get stronger and faster.
Why is it in the news now?
Drug testers have only recently worked how to test for THG, but they suspect people have been using it for months already.
They're using their new test for THG on drugs tests given much earlier in the year, and are catching lots of people out.
Why couldn't they test for it before?
They didn't know it existed until recently when a top US coach secretly sent some to the drugs testers and said lots of athletes were using it.
Once they had the sample of the drug they worked out how to test for it.
Who have they caught?
In the UK only sprinter Dwain Chambers has been named.
It's been reported that as many 20 American athletes may have been using THG at the US Athletics Championships, their trial for August's World Championships.
Some of them may have gone on to compete - and win medals at the championships - while others may even be world record holders.
If they test positive, any medals or records they set while using the drug will be taken away from them.
Why don't we know who they all are yet?
When an athlete is tested for drugs, the testers take two samples, and so far only the first alpha or A tests have been carried out.
If the beta or B tests of those athletes test positive for THG too, the names will be made public.
What happens to athletes who test positive?
They will face a minimum of a two-year ban from all athletics.
The British Olympic Association also add their own ban, stopping athletes who test positive from ever competing in an Olympics ever again.
Where do athletes get THG from?
Most suspicion is being aimed at a place called BALCO in San Francisco in America. The United States Anti-Doping Agency says that's where THG comes from.
BALCO supplies lots of top athletes with all kinds of supplements, and is in trouble with the US courts for allegedly avoiding paying their taxes.