Ethanol derives from the fermentation of naturally occurring sugars. The International Union of Pure and Applied Chemistry (IUPAC) nomenclature for the chemical compound ethanol is a structural composition consisting of a pair of carbon atoms with an alkyl group coupled with an -OH functional group; the -OH group makes it chemically an alcohol. Ethanol, unlike methanol, butanol, propanol, etc., can be consumed by humans without dying in its purest form. 

Ethanol's uses include:

1. Ethanol in lieu of fomepizole in methanol poisoning [1]  

2. Ethanol in lieu of fomepizole in ethylene glycol poisoning [2] 

3. As a hand sanitizer for the prevention of healthcare-associated infections (HCAI) [3] 

4. Ethanol lock therapy for reducing the incidence of bloodstream infections acquired through IV catheters [4]

Ethanol as an embolic, sclerosing and an ablative agent:  [5]  

1. As an ablative agent in hypertrophic obstructive cardiomyopathy by controlled septal infarction [6]

2. As an embolic agent for embolization of arteriovenous malformations [7] 

3. As a sclerosing agent to reduce pain in musculoskeletal hemangiomas, as an alternative to surgery or to reduce the morbidity associated with the tumor resection [8] 

4. As a sclerosant for lymphatic malformations occurring in the neck and the mediastinum under the guidance of CT [9] 

5. As a sclerosing agent in the treatment of symptomatic simple renal cysts [10] 

6. As an ablative agent against thyroid cystic nodules [11] 

7. As an ablative agent in endoscopic ultrasound (EUS) guided ablation of a single insulinoma [12]

8. As an ablative agent for hepatocellular carcinomas [13] 

9. Ethanol injected peripherally in abetting pain in trigeminal neuralgia [14]  

10. For neurolysis of brachial plexus pain associated with cancers [15]  

11. For neurolysis of splanchnic nerve to decrease cancer-associated upper abdominal pain [16] 

As an antidote for: 

1. Methanol poisoning: Methanol per se is not as toxic as its metabolized counterpart, i.e., 'formic acid.' Methanol is acted upon by alcohol dehydrogenase to convert it into 'formaldehyde.' Aldehyde dehydrogenase (ADH), in turn, acts on formaldehyde to convert it into formic acid, and this metabolite brings about the deleterious effects of methanol poisoning such as vision loss and Parkinsonian movements. Ethanol metabolism is also by ADH, and its affinity for ADH is significantly higher than methanol; hence ethanol administration results in competitive inhibition of methanol, slowing down the formation of formic acid. 
2. Ethylene glycol poisoning: Ethylene glycol, very much like methanol, gets metabolized by alcohol dehydrogenase (ADH) in the liver to form glycoaldehyde. Glycoaldehyde subsequently converts to glycolic acid and oxalic acid. Administration of ethanol results in competitive inhibition of ethylene glycol from binding to ADH as the former will saturate the enzyme. 

As a hand sanitizer and preventing sepsis: 

1. Many credible surveys report the global burden of healthcare-associated infections (HCAI). Alcohols have an inherent antimicrobial property, which works by denaturing and coagulating proteins, disrupting their cell wall, and killing them. Ethanol is highly efficient against viruses and can be used in adjunct with other alcohols to obtain a powerful synergistic effect against microorganisms. 
2. Ethanol lock therapy (ELT) follows the ideals mentioned earlier of killing bacterias and disrupting their biofilms. This concept is mainly useful for patients receiving total parenteral nutrition. An ethanol lock created in the lumen of the IV catheter, both kills and inhibits the growth of bacterias, thus reducing  'catheter-related bloodstream infections' (CRBSI) markedly. 

 As an ablative/sclerosing/embolic agent: 

1. Ethanol works by damaging the endothelial lining and dehydrating the cells, thereby resulting in fibrotic changes. As a consequence, tumor/hyperplastic growth suffers an ischemia and size reduction.[17]

For methanol and ethylene glycol poisoning, ethanol administration can be either through an IV line, a nasogastric tube, or even orally. Still, an IV ethanol infusion is always preferable to other routes. A plasma concentration of methanol exceeding 20 mg/dL is a reference for starting ethanol therapy.

Commercially available, 10% ethanol solution dosing is as follows:

• 7.6 mg/kg IV x 1 if the patient's ethanol level is zero, decrease if ethanol is present in the bloodstream
• Maintenance infusion is 0.83 mL/kg/hour for non-drinker, 1.96 mL/kg/hour for chronic alcohol consumers

This regimen can occur in an intensive care setting with the plasma concentrations of alcohol requiring hourly monitoring until reaching a desired plasma ethanol concentration of 100 mg/dL, following which the plasma sampling frequency is reducible to 2 to 4 hours. One must keep in mind that the FDA approved IV fomepizole for methanol/ethylene glycol poisoning, and it is a specific inhibitor of ADH. It has a far superior affinity for ADH than ethanol. Even though fomepizole is a superior antidote in comparison to ethanol, the availability and the limited knowledge of fomepizole has resulted in the continuance of ethanol infusions as a treatment in these cases.[18][19] 

Alcohol-based hand sanitizers (ABHS) are time-proven weapons for preventing nosocomial infections. However, the efficacy of the same against neutralizing microorganisms rests on the effectiveness of its use. Several studies dedicated to establishing complete disinfection on the surface of the hands have concluded that an amount of 2.4 to 3 mL is the recommendation with an application time of minimum 25 to 30 seconds to establish maximum disinfection.[3] 

Ethanol lock therapy (ELT) uses 70% ethanol over a minimum of 4 hours, reaching a maximum of 6 hours in conjunction with the regular antibiotic therapy has significantly brought down the rate of catheter-related bloodstream infection (CRBSI). This therapy is especially useful when there have been no reports of bacterias being resistant to ethanol. 

Ethanol, as a chemical ablative/sclerosing agent, has been put to use rigorously, in treating varying neoplasms, cysts, vascular malformations, and even chemical neurolysis. Ethanol is injected into the parenchyma/lesion of the specific organ. It can utilize the guidance of endoscopic ultrasound, or in some cases, the injection can be under CT guidance.  

When ethanol converts to acetaldehyde, NAD+ reduces to NADH. NAD+ serves as an electron carrier and donates 2 electrons in this reaction. This reaction increases the NADH/NAD+ ratio in the hepatocytes. This increase in ratio causes:

Ketoacidosis: Via inhibition of the citric acid (TCA) cycle, thereby increasing the levels of acetyl-CoA. This increased acetyl-CoA is shunted into the ketogenesis pathway resulting in ketoacidosis. 

Fasting hypoglycemia: The increased ratio inhibits the conversion of malate to oxaloacetate (OAA). This inhibition changes the reaction equilibrium by starting to convert OAA back to malate, impairing gluconeogenesis because the meager amount of OAA is left to convert to phosphoenolpyruvate (PEP) for gluconeogenesis.

Hepatic steatosis: The increased NADH/NAD+ ratio increases the synthesis of fatty acid and glycerol-3-phosphate, thereby increasing the levels of TAGs resulting in fatty liver. 

Ingested ethanol mainly metabolizes in the liver; hence hepatic damage is profoundly seen in chronic alcoholism. This damage includes steatosis, cirrhosis, and hepatic carcinoma. Other organs/organ systems involved are:

1. Gastrointestinal System: gastritis, malabsorption, carcinomas 
2. Cardiovascular System: hypertension, cardiomyopathies, arrhythmias 
3. Pancreas: through acinar damage progressing to pancreatitis
4. Renal System: glomerulonephritis and AKI 
5. Reproductive System: infertility, premature birth, SGI babies and fetal alcohol syndrome
6. Breast carcinoma 
7. Neuro/psychiatric consequences: stroke, major depression, meningitis, alcoholic cerebellar degeneration [20]

In cases of methanol/ethylene glycol toxicity: 

The loading and the maintenance dose should be carefully monitored in an intensive care setting to prevent the patient from any toxic manifestation of the antidote-ethanol itself. Furthermore, ethanol must not be administered concurrently with fomepizole as the later will inhibit the metabolism of ethanol in the same way as it inhibits the metabolism of methanol/ethylene glycol. This inhibition will result in an exaggerated rise in ethanol level in the body as its half-life is now prolonged, and will inadvertently result in the manifestation of the intoxicating nature of ethanol. Contraindications to ethanol also include pregnancy and breastfeeding.

For ABHS:

Though there is no clearcut contraindication in any particular medical setup, it has been well established that ABHS doesn't kill the spores of several microorganisms, including the likes of Clostridium difficile. It is also not effective against certain protozoan oocysts, for example, cryptosporidium. ABHS has also been inefficient against a certain non-enveloped virus-like Norovirus.[3]

As an Ablative Agent:

Percutaneous ethanol injection is contraindicated when:

1. Prothrombin time (PT) is less than 40% of the normal.
2. In thrombocytopenia (count < 40,000/ mm^3) [21]