Robo, short for Robitussin, is slang for any DXM-containing product, not just Robitussin brand items. These include Delsym, Vicks, or Zicam. Ingesting recreational doses causes you to robotrip.
Doses are divided into 4 (or 5 plateaus). The lower plateaus cause euphoria and some closed eye visuals (CEVs). The upper plateaus cause dissociation, CEVs, and sometimes open eye visuals (OEVs). The 5th plateau (Plateau Sigma) is reached by redosing multiple times.
An easily accessible drug, it is a favorite for high school kids. Because of this, it has a label as a "kiddie drug". In reality, it is a strong dissociative hallucinogen that produces a very unique trip.
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