This happened a long time ago. I had gotten really sick with a bronchial infection or something, and my doctor gave me prescription cough syrup with codeine in it. I must have been pretty sick because it was a big bottle. I probably used like half of it or less, and left it in the cabinet.
My girlfriend at the time came to stay with me for a few months, and she had a cough or something, she just thought it was normal cough syrup, and took a bunch of it. She didn't know what it was, so I jokingly explained to her that she had just ingested "Lean", the slang term for codeine cough syrup.
A few days later she had to go to the doctor for something where they had to give her a blood test as part of her routine workup because of some other medicine she was taking. They found the codeine in her system and asked her, and she said something like "Oh yeah I took some 'Lean'" which they were like WTF to. She thought that was what it was actually called, so once they straightened it out and she told them the situation, there was a lot less concern.