**Question:** N-acetyl-cysteine is antidote for: September 2012

**Core Concept:** N-acetyl-cysteine (NAC) is a precursor to the antioxidant glutathione and is used to increase its levels in the body. It is also involved in the synthesis of cysteine, an essential amino acid for protein synthesis.

**Why the Correct Answer is Right:** N-acetyl-cysteine is primarily used as an antidote for acetaminophen (paracetamol) overdose. Acetaminophen (paracetamol) is a widely used analgesic and antipyretic drug that can cause severe hepatotoxicity and liver failure if taken in excess. NAC acts as a glutathione precursor, increasing the liver's defense mechanism against acetaminophen-induced oxidative stress. It helps in the detoxification process by competing with acetaminophen for binding to the liver enzymes involved in acetaminophen biotransformation, thereby preventing the formation of toxic N-acetyl-p-benzoquinone imine (NAPQI) and reducing the risk of acetaminophen-induced liver damage.

**Why Each Wrong Option is Incorrect:**

A. N-acetyl-cysteine is not an antidote for methotrexate overdose. Methotrexate is a folic acid antagonist used for cancer treatment and autoimmune disorders, but its excessive intake can lead to toxicity and renal failure. NAC's primary role is in acetaminophen overdose management, not methotrexate poisoning.

B. Although NAC has antioxidant properties, it is not specifically designed as an antidote for cyanide poisoning. Cyanide poisoning primarily involves inhibiting mitochondrial respiration and leading to intracellular hypoxia, which can cause tissue damage and organ failure. Antidotes for cyanide poisoning, like hydroxocobalamin (Cyanokit) and hydroxocobalamin, target the nitric oxide-hydroxyl radical reaction rather than relying on antioxidants like NAC.

C. N-acetyl-cysteine is not an antidote for acetaminophen poisoning. As mentioned earlier, NAC primarily focuses on preventing acetaminophen hepatotoxicity by increasing liver glutathione levels, not treating the poisoning itself.

D. N-acetyl-cysteine is not an antidote for carbon monoxide poisoning. Carbon monoxide poisoning involves the binding of carbon monoxide to hemoglobin, leading to reduced oxygen transport and cellular hypoxia. Antidotes like hydroxocobalamin (Cyanokit) and hyperbaric oxygen therapy are used for carbon monoxide poisoning, not NAC.

**Core Concept:** N-acetylcysteine (NAC) is a precursor to cysteine, which is an essential amino acid for protein synthesis, and serves as a precursor to glutathione, a potent antioxidant. NAC has been found to have anti-inflammatory and anti-apoptotic effects, making it beneficial in treating acute lung injury and acute respiratory distress syndrome (ARDS).

**Why Each Wrong