It is possible, my best friend died this way. She was on a psychiatric unit. She tried it several times before she was successful, every other time she was caught by the nursing staff but the final time they didn't check on her until it was too late. She used to wet the toilet paper (by running it under the tap) before putting in her mouth/down her throat. I've tried it once, out of curiosity. I wanted to know what it was like for her. It made me retch, I have sensory issues and couldn't stand the feeling of the toilet paper in my mouth. But if you're determined enough and have nothing else at your disposal, people can ctb this way.