N-acetylcysteine is a precursor of an amino acid (L-cysteine), which is itself a component of an antioxidant. It may help protect against oxidative stress by maintaining or increasing antioxidant levels in the body. Some clinical trials testing formulations that included N-acetylcysteine have found modest improvements in cognitive function, but clinical trials testing it alone have shown less robust results. N-acetylcysteine supplements are generally regarded as safe when taken at recommended doses.
No randomized controlled trials have examined whether N-acetylcysteine alone can prevent cognitive decline or dementia, though some evidence exists for combination therapies.
Our search identified:
• 8 randomized controlled trials (3 testing cognitive function, 2 in Alzheimer's patients, 1 in patients with mild cognitive impairment, 1 in Parkinson's disease patients, and 1 in schizophrenia patients)
• Numerous preclinical studies
No studies have examined prevention of cognitive decline or dementia in humans with N-acetylcysteine alone. In a clinical trial of people with mild cognitive impairment, daily consumption of a nutraceutical formulation containing 600 mg of N-acetylcysteine (along with folate, alpha-tocopherol, vitamin B12, S-adenosyl methionine, and acetyl-L-carnitine) for 6 months was associated with improvement in dementia rating scale and executive function . However, no direct comparisons were made between the nutraceutical treatment group and the placebo group, making the results inconclusive. Another clinical trial using the same nutraceutical formulation for three months did find that it was associated with improved verbal learning compared to the placebo group . And both the placebo and nutraceutical groups improved further during a three-month extension during which everyone received the formulation.
However, studies testing the effects of N-acetylcysteine alone found only limited evidence for cognitive protection . (These studies were conducted in patients with schizophrenia or bipolar disorder.)
A clinical trial in Alzheimer's patients found that a nutraceutical formulation containing N-acetylcysteine resulted in significant improvements in some cognitive functions compared to the placebo group . However, a clinical trial testing it alone showed less robust effects. The patients receiving N-acetylcysteine performed better than the placebo group on a letter fluency task but not on other cognitive tests .
N-acetylcysteine supplements are generally regarded as safe for most adults when taken at standard doses. The nutraceutical formulation used in older people and those with mild cognitive impairment was well-tolerated and no serious side effects were reported for any of the 300+ participants . N-acetylcysteine should not be administered with nitroglycerin, as the combination can cause severe hypotension  and headache . N-acetylcysteine interacts with inhaled insulin (Afrezza™, Exubera™) and may affect the absorption of insulin into the blood stream .
NOTE: This is not a comprehensive safety evaluation or complete list of potentially harmful drug interactions. It is important to discuss safety issues with your physician before taking any new supplement or medication.
N-acetylcysteine is available as a dietary supplement typically containing 600 mg per capsule. A nutraceutical formulation that showed some cognitive benefits in people with mild cognitive impairment or Alzheimer's disease contained 600 mg of N-acetylcysteine, along with other compounds (i.e., folate, alpha-tocopherol, S-adenosyl methionine, acetyl-L-carnitine) .
Information including possible side effects of N-acetylcysteine at Drugs.com
Quality Control of Sources: United States Pharmacopeial Convention (USP) and FDA Information on Dietary Supplements offer information on the quality of specific supplements and can assist in finding a trusted brand.
The ADDF's evaluation of N-acetylcysteine in The Journal of Prevention of Alzheimer's Disease.