N-Acetyl-L-cysteine (NAC) is a white crystalline powder that melts in the range of 104–110°C and has a very slight odor. It is a natural sulfur-containing compound that is produced in living organisms from the amino acid cysteine. It is involved in the intracellular synthesis of a chemical called glutathione (GSH). Cells (particularly liver cells) use GSH to detoxify chemicals by making them more water soluble and thus easier to excrete from the body. NAC is also a powerful antioxidant. 

Uses

N-Acetyl-L-cysteine is a medication that is used to treat paracetamol (acetaminophen) overdose, and to loosen thick mucus in individuals with chronic bronchopulmonary disorders like pneumonia and bronchitis. It has been used to treat lactobezoar in infants. It can be taken intravenously, by mouth, or inhaled as a mist. Some people use it as a dietary supplement. 

Pharmacodynamics

N-Acetyl-L-cysteine serves as a prodrug to L-cysteine, a precursor to the biologic antioxidant glutathione. Hence administration of acetylcysteine replenishes glutathione stores. 

Glutathione, along with oxidized glutathione (GSSG) and S-nitrosoglutathione (GSNO), have been found to bind to the glutamate recognition site of the NMDA and AMPA receptors (via their γ-glutamyl moieties), and may be endogenous neuromodulators. At millimolar concentrations, they may also modulate the redox state of the NMDA receptor complex. In addition, glutathione has been found to bind to and activate ionotropic receptors that are different from any other excitatory amino acid receptor, and which may constitute glutathione receptors, potentially making it a neurotransmitter. As such, since N-acetylcysteine is a prodrug of glutathione, it may modulate all of the aforementioned receptors as well.

Glutathione also modulates the NMDA receptor by acting at the redox site. 

N-Acetyl-L-cysteine also serves as a precursor to cystine, which in turn serves as a substrate for the cystine-glutamate antiporter on astrocytes; hence there is increasing glutamate release into the extra-cellular space. This glutamate in turn acts on mGluR2/3 receptors, and at higher doses of acetylcysteine, mGluR5. 

Acetylcysteine also possesses some anti-inflammatory effects, perhaps via inhibiting NF-κB and modulating cytokine synthesis.

Side effects

Common side effects include nausea and vomiting when taken by mouth. The skin may periodically become red and itchy with any route of administration. A non-immune type of anaphylaxis may also occur. It appears to be safe in pregnancy. For paracetamol overdose, it works by increasing the level of glutathione, an antioxidant that can neutralise the toxic breakdown products of paracetamol. When inhaled, it acts as a mucolytic by decreasing the thickness of mucus.