The Notting Hill star, who portrays infamous drug smuggler Howard Marks in upcoming biopic Mr. Nice, is adamant the criminalisation of marijuana is "idiotic" and creates more problems.
And Ifans is urging the British government to consider controlling pot in the same way as tobacco and alcohol.
He tells Britain's FHM magazine, "I agree with Howard on this. You look at any culture and prohibition has invariably been an unmitigated failure. It is just idiotic to criminalise any substance, I think. It needs to be controlled, managed.
"If you legalise something then you use the taxation from the sale of that drug to help people who become addicted to those substances. It is not going to go away. The war on drugs is being lost on a daily basis.
"In the past we have had a little smoke, the odd toke. I didn't inhale though. Howard still has a smoke. Everyone has a little smoke once in a while, you know. It's one of life's little pleasures."