A woman presents with persistent vomiting and a negative ethanol level. What substance is most likely responsible?

In cases of persistent vomiting with a negative ethanol level, the substance most likely responsible is isopropyl alcohol. Isopropyl alcohol, also known as isopropanol or rubbing alcohol, is commonly encountered and can lead to symptoms like vomiting when ingested, especially in significant amounts. Unlike ethanol, which is checked in the ethanol level test, isopropyl alcohol does not generate a positive result on this test, making it a relevant consideration when ethanol levels come back negative.

Methanol and ethylene glycol are also toxic alcohols that can cause vomiting and other serious symptoms; however, they typically present with additional signs such as metabolic acidosis or specific visual disturbances in the case of methanol poisoning. While they can be serious, isopropyl alcohol is more commonly associated with straightforward presentations of nausea and vomiting without the specific sequelae seen with the other two.

Additionally, ethanol itself is ruled out as a cause since the ethanol level is negative. Therefore, isopropyl alcohol stands out as the most likely candidate causing the symptoms in this particular case.