Chorus: "I'm in their second hand smoke / Still just drinking canned Coke / I don't need a Xanny to feel better / On designated drives home / Only one who's not stoned / Don't give me a Xanny, now or ever"
Meaning: It implies the confrontation to the drugs by the character (perhaps the same singer, Eilish). With the first line, is seeable what is the situation the character is in is like, this is negative, since it's having a negative impact on her or him (Secondhand smoke is smoke inhaled involuntarily by a non-smoking person). Continuing, we can observe that the character continues to drink “canned Coke” as opposed to using the drug, Xanax, even, affirms to her or his friends that doesn’t “need a Xanny to feel better”. With this, apparently, the character is the only one who refrains from using these drugs and as a result, is the only one who can drive her friends home as a sober individual. Thus, the character has seen the impact of drugs first hand. Therefore, he/she doesn’t want to be given “a Xanny, now or ever.”