My 'd'h used to smoke fags and casually would smoke pot. He stopped both and had been off them for a couple of years. We got married had 2dds, all was great.
He started smoking fags again and an old friend of his came back into our lives. Dh has an unsociable job, no hobbies out of the home, so once a week he has been going to this friends for an evening, friend is unwell and doesn't leave the house much but smokes pot. I realised that dh was smoking again when there and we have argued about this with him promising it's not a problem 
etc etc. Now I think it's escalated and although he doesn't smoke around the house or me I suspect he is paying for weed again.
Short of going through his phone I can't prove this but we dont have much money and his boss pays cash (all through the books) just not into the bank. We seem to run out quickly and I know he's keeping upwards of £20 a week to himself.
I have tried to talk to him and get nowhere as it's just denial and the more I push the more defensive he becomes, so we just go round in circles.
I can cope (just) with him smoking fags again but i resent our family money (we've always had everything joint and all income is ours together) being used for that.
I don't know what to do, my parents help us out a lot and I feel like it's a slap in their face if I have to ask them for financial help but he's wasting money.
Sorry it's so long just need to get it out, don't have any real life support, my parents know about the smoking fags but I can't tell them about the rest yet, and I think he'd leave before he'd stop smoking completely.