Ethanol Intoxication: This is often the most common cause of altered mental status due to its widespread use and the frequent presentation of intoxicated patients to emergency departments. Ethanol can cause a range of effects from mild impairment to coma, depending on the blood alcohol concentration.

Opioid Overdose: Opioids, including prescription medications and illicit drugs like heroin and fentanyl, can cause significant respiratory depression and altered mental status. The opioid epidemic has increased the prevalence of opioid-related emergencies.
Benzodiazepine Overdose: Similar to opioids, benzodiazepines can cause significant sedation and altered mental status, especially when used in combination with other central nervous system depressants.
Cocaine or Amphetamine Intoxication: These stimulants can cause agitation, aggression, and altered mental status. Their use is common, and the effects can be severe, especially in cases of overdose or when combined with other substances.
Carbon Monoxide Poisoning: This can occur from inhalation of fumes from improperly ventilated combustion sources. It's a significant concern in cases of suspected overdose or altered mental status in individuals found in enclosed spaces with running engines or faulty heating systems.

Salicylate Toxicity: Aspirin overdose can lead to severe metabolic acidosis and altered mental status. It's crucial to consider this diagnosis due to the potential for severe outcomes if not promptly treated.
Tricyclic Antidepressant (TCA) Overdose: TCAs can cause significant cardiac and neurological toxicity, including altered mental status, seizures, and cardiac arrhythmias. The potential for severe and life-threatening complications makes this a "do not miss" diagnosis.
Gamma-Hydroxybutyrate (GHB) Overdose: GHB can cause deep sedation and altered mental status. Its use is less common but can be deadly, especially when combined with other substances.

Methanol or Ethylene Glycol Poisoning: These substances, found in antifreeze and windshield washer fluid, can cause severe metabolic acidosis and altered mental status. While less common, they are critical to diagnose due to the specific antidotes required (fomepizole or hemodialysis).
Heavy Metal Poisoning (e.g., Lead, Mercury): Exposure to heavy metals can occur through occupational or environmental means and can lead to a range of neurological symptoms, including altered mental status.
Organophosphate or Carbamate Poisoning: Typically seen in agricultural workers, these pesticides can cause cholinergic crisis leading to altered mental status among other systemic effects.