Just to be the odd one - and please don't take this as me saying that clients on drugs are all pussycats really, but one of my regs is a coke dealer, who didn't give any hint of this in advance and the first time I met his was an out call to his place.
I had security on standby as he didn't have a landline, he lives in a newly built flat that google maps could only give a rough idea of the location of, and his town is an hour round trip from the city in the back end of beyond. (looking back I have no idea why I went lol!)
So he booked me for 4 hours, picks me up at the train station in his little boyracer-mobile and takes me back to his. He's young, fit, tanned and rather attractive if bordering on chavvy.
Then when we get to his he goes to pay me from a HUGE brown envelope stuffed with ?20s, and I noticed a few notes on the kitchen counters all rolled up. He then removes the spot light from his ceiling and pulls out a bag of cocaine!
I have friends who smoke weed but thats as close to drugs as I get, I don't even smoke ciggys. I told him this when he offered me some and he laughed his ass of and told me I was so cute and that he's glad I'm clean and went on to expound the evils of drugs, while doing lines off the bench with a ?20 note!
The booking went on and we both had a lot of fun, so much in fact that he was knackered after 2 and a half hours but he paid my full 4 hour fee, plus ?60 because he knew I had misgivings about visiting him.
In short he was a lovely guy all around and I'm glad I met him as he is one of my nicest clients. I'm not saying anyone should take insane risks but this guy was over the moon that I didn't walk out and let him enjoy his hobby without judging him, and I was rewarded for that with his regular business.
I think you have to go with your gut, and if you turn out to be wrong finding a way of getting out of the situation is priority number 1