and is used for physical pain

Nate Diaz withstood some incredible punishment at UFC 202 but may be about to take some more as he faces an investigation from the US anti-doping agency.

The Stockton fighter was seen smoking from a vape pen at the post-fight press conference. When asked about it, Diaz explained the substance was called CBD oil which is similar to medical marijuana and used to ease physical pain and trauma.

Cannabinoids are on a list of banned in-competition substances and are a group of closely related compounds found in cannabis. The definition of 'in competition' is six hours before the fight and six hours after it, meaning Diaz's decision to light up at the post-fight press conference could come back to haunt him.

Nate Diaz said the CBD oil was to ease his pain and help the post-fight healing process

Diaz was still 'in competition' when he decided to light up the vape pen on stage

The UFC are partnered with the US anti-doping agency and have cracked down on use of substances on the banned list with numerous fighters receiving bans.

Given that Diaz explained what he was doing in front of media gathered from around the world, there was little chance the indiscretion would not be brought to the attention of USADA.

There are different guidelines from both the Nevada Athletic Comission and USADA regarding testing procedures. For the NAC the process ends with the post-fight sample being given. Whereas the UFC-USADA agreement means the testing period is extended to six hours after the bout.

Diaz lost a hard fought decision to Conor McGregor and was visibly battered after the contest.

The Stockton fighter now faces uncertainty over any sanctions which might be handed out

He explained that the CBD oil was to ease his pain after the fight and said it: 'helps with the healing process and inflammation.'

If Diaz is found guilty of using a banned substance 'in competition' he could face a suspension.