Mine was addict to cocaine and only did it behind my back we was together for 5 years before I found out. He even let me have a child with him during his active addiction.
I think that he is being honest when he is using is a good thing. When the lies start is when things get really bad, hard. Mine went to the extreme of saying he was working away and checking himself into hotel rooms and sleeping in his car to avoid being caught.
What I have definitely learnt is that your cannot force an addict and they will only stop when they are ready themselves and effort you put into attempting to force them is completely wasted.
I ended up separating from mine for the sake of my own MH as well as my childs with no contact completely before he decided to sort his life out, after an very poor attempt at suicide, He is now 20 days into being sober and living with his parents at the moment but it took complete cut off from me and no contact with either myself or our child for almost a month before he even considered to straighten himself out.