The side effects of N-acetyl-L-cysteine depend on the route of administration of the drug. If taken orally this drug can cause nausea, vomiting, diarrhea or constipation at most. According to WebMD, in certain populations of people such as those with asthma, this drug may cause the side effect of bronchospasms. N-acetyl-L-cytsteine is used to clear out mucus in the respiratory system, making it a good treatment for cystic fibrosis.
The side effects caused by N-acetyl-L-cysteine are minimal in comparison to the benefits this drug provides to those with acetaminophen poisoning. According to the Journal of Clinical Investigation as archived with the National Center for Biotechnology Information, N-acetyl-L-cysteine is considered an effective antidote for acetaminophen poisoning because it prevents the reaction that is responsible for Tylenol toxicity. This reaction between the toxic metabolite of acetaminophen, N-acetyl-p-benzoquinoneimine, and liver cells is stopped by N-acetyl-L-cysteine binding to the toxic part of acetaminophen instead. Keeping the toxic part of acetaminophen away from liver cells helps prevent acetaminophen poisoning.
According to WebMD, N-acetyl-L-cysteine does have an unpleasant odor which makes it difficult to take. This characteristic odor may hold some responsibility for the side effects of nausea and vomiting. According to Amazing Wellness Magazine, the high sulfur content of this drug makes it smell like urine.