To me, the drugs may or may not be a deal breaker depending on a variety of factors. If he planned ahead and stayed elsewhere, didn't spend much money and able to function normally (no HM issues) the rest of the time, every 6 months doesn't seem terrible to me, but I know quit a few people who take drugs, and although it's not for me, I know it's not the black/white evil that lots of people think it is.
On the other hand, I know what it's like to be the child of a drug addict (actual addict, not recreational 6month-er). If there was any possibility of actual dependance, he would be out of my life like a shot. Believe me, it is horrific growing up knowing that your dad is a useless, dirty, incapable, thieving addict (in my case, obviously not the same for all cases.)
But, as everyone else says, you need to decide for yourself where the line (no pun intended) is. The lying, I wouldn't be happy with, but can see that you're perhaps giving him mixed messages about how much it means to you, which leads to the lying.