Seven soldiers have been dismissed from the 1st Battalion of the Welsh Guards after failing routine drugs tests.
The Army has strict rules on drugs
Commanding officer Ben Bathurst told Good Morning Wales the seven "had to go" after they admitted taking drugs.
Four more Welsh Guards and 14 troops from the Green Howards are also facing expulsion after failing the tests.
The Ministry of Defence said action would be taken against all those who tested positive - 16 for Class A drugs, one for Class B and eight for Class C.
'No hiding place'
Lt Col Bathurst said dismissal was "what all soldiers are aware can happen".
He said: "My view is that drug use is so incompatible with army life. We depend upon fitness, alertness and team work and it just does not mix in.
"The reason why I dismissed all seven, who admitted it straight away, is that there are no second chances. They all know the rules, they all know my policy on zero tolerance and they had to go."
An MoD spokesman said: "Appropriate internal action is being taken against all the soldiers and it is likely that they will be discharged in line with army policy."
He added that there was no suggestion the behaviour of those who had taken drugs was reflected across the Welsh Guards, whose ceremonial colonel is Prince Charles.
A spokesman for the army in Wales said military drug tests were carried out without warning.
"These drug teams just turn up, you don't know when they are coming.
"They will do any rank. It's as simple as that. There is no hiding place."
The 1st Battalion Welsh Guards, based in St Athan, south Wales, has recently returned from a tour in Iraq and has also been sent to Bosnia and Northern Ireland.
The MoD spokesman said Welsh Guards had demonstrated their "operational effectiveness" during a "successful" tour of Iraq.
The Green Howards, which has bases in North Yorkshire and Chepstow, returned from a two-year tour of Northern Ireland in 2004.
Five guardsmen and 11 Green Howards tested positive for Class A drugs, one Green Howard for Class B and the rest for Class C drugs.
The MoD spokesman said the rate of positive tests was 0.9% in the Army, compared to 5% of civilians.