Tussin
Tussin is a slang word for cough medicine, derived from the common cough and cold product, Robitussin.
Susan Discusses Her Daughters DXM Use:

"I didn’t realize that this was a problem with Shannon, for a very long time. She hid it very well. When I used to go looking for her, she would be in her car and she’d be stoned. She’d be high on
DXM. It took about a good year before I realized that this is really a problem -- that she’s got some kind of addiction going on here. And it’s to something like cough medicine, to
Robitussin.
"In the beginning, when Shannon started taking DXM, she wasn’t as careful as she was as it went on. I would find the little caps from the Robitussin bottle, empty bottles stuffed behind her bed, and the gel caps too -- the empty little blisters from the pill packets would be all over or they’d be in her car. I’d look around and see a little bottle of Robitussin that’s empty in the house. She didn’t think consciously, that we were actually looking and searching for things. As time went on, she became more and more careful, however. I was going through her things when I suspected it was a real problem. And it got to the point where she would leave the house, go and buy it, drink it, throw it out someplace, and we’d never find a telltale sign. There are telltale signs.
"We found out Shannon was drinking Robitussin to get high, purely by accident. My husband couldn’t wake her up one morning. She was almost anesthetized or something. He kept shaking her saying, ‘Shannon, get up, get up, what’s wrong with you? What’s wrong with you?’"
Watch Susan’s video.