NAC Side Effects and Safety: Is NAC Dangerous?

Is NAC dangerous? For most healthy adults taking it by mouth at the doses used in studies, the research says no. Reviews describe NAC (N-acetylcysteine) as generally well tolerated, and the most common side effects are an upset stomach and its sulfur smell and taste. A few groups do need to be careful, mainly people on blood thinners or nitroglycerin. This guide covers NAC side effects, who should check with a clinician first, how much is too much, and what the FDA has said about NAC.

It is educational, not medical advice.

Key Takeaways

• Generally well tolerated. A clinical review describes oral NAC as generally well tolerated below 1,200 mg a day, and the FDA said in 2022 that its initial review had not revealed safety concerns with NAC in dietary supplements.
• Most common side effects: nausea, vomiting, diarrhea, and an unpleasant sulfur smell and taste.
• Higher doses: a review of 41 studies using 600 to 3,000 mg a day found a similar safety profile at high and standard doses, with stomach symptoms no more common than in control groups.
• Check first if you take blood thinners or nitroglycerin, are pregnant or breastfeeding, or are having cancer treatment.
• Nasal NAC uses a much smaller dose; its side effects are usually brief stinging, a runny nose or a sulfur smell.

NAC Side Effects at a Glance

Most people tolerate NAC well. Reviews describe oral NAC as generally well tolerated below 1,200 mg a day, and the side effects people do notice are usually mild. Here is the short version; each row is covered in more detail below.

Group	NAC side effects	What helps
Common (oral)	Nausea, vomiting, diarrhea, and a sulfur smell and taste; usually mild	Take it with food, split larger doses and start low
Less common (oral)	Mouth sores, drowsiness, a runny nose and the other rarer reports listed below	If you feel drowsy, avoid driving and talk with your healthcare provider
Nasal spray	Brief stinging, a runny nose for a few minutes, a sulfur smell	Much smaller dose than capsules; stop if you get repeated nosebleeds
Who should check first	People on blood thinners or nitroglycerin, with a bleeding disorder, having cancer treatment, pregnant or breastfeeding, or taking prescription medication	Ask your doctor or pharmacist before starting

Comparing NAC with glutathione itself? Our guide to glutathione side effects covers how the finished molecule is tolerated, and nasal sprays vs capsules explains how the two formats differ.

Is NAC Dangerous?

NAC is the amino acid cysteine with an acetyl group attached. Your body uses the cysteine to make glutathione, your cells' main antioxidant (more on our NAC ingredient page and in NAC vs glutathione). It has been used in medicine since the 1960s, so there is a long record of how people tolerate it. For glutathione itself, see our glutathione ingredient page.

That record is reassuring. A 2021 clinical review concluded that NAC appears to be well tolerated with minimal side effects when used as a supplement, and generally well tolerated in oral doses below 1,200 mg a day (Schwalfenberg, 2021). In a small study of 30 healthy volunteers who took a single 600 mg dose and then 600 mg twice a day for three days, two people reported a total of three side effects, including a headache, and the authors described NAC as having a favorable tolerability profile (Papi et al., 2021).

Generally well tolerated does not mean right for everyone. NAC can interact with some medicines, and a few groups should talk with a clinician first. Those are covered below.

What the FDA has said about NAC

Some of the worry about NAC comes from its legal status, not its safety. The FDA's position is that NAC is excluded from the definition of a dietary supplement. In August 2022, the FDA issued final guidance saying it would use enforcement discretion for certain NAC products labeled as dietary supplements, and said that its initial review "has not revealed safety concerns" with NAC used in or as a dietary supplement (FDA, 2022). For what NAC does and what studies show, see NAC benefits.

Common NAC Side Effects

When NAC is swallowed, the side effects people notice most are in the stomach:

• Nausea.
• Vomiting and diarrhea.
• An unpleasant smell and taste. NAC contains sulfur, which gives capsules and powder their sulfur smell.

These are usually mild. In a review of 41 studies, mostly of oral NAC, at 600 to 3,000 mg a day, the safety profile was similar at high and standard doses, and stomach symptoms were no more common than in the control groups (Calverley et al., 2021).

How to make side effects less likely

• Take it with food if it upsets your stomach.
• Split larger doses, for example 600 mg in the morning and 600 mg in the evening, rather than one large dose.
• Start low and stay within the label.
• Try a different form. Effervescent tablets dissolved in water are easier for some people to take than capsules; our best NAC supplement guide compares the forms.

Rare and Serious Reactions

Rarer reported side effects include mouth sores, drowsiness, a runny nose and coughing up blood (Schwalfenberg, 2021).

The more serious reactions in the literature mostly come from high doses given by IV in hospital, not from capsules. With intravenous NAC at 150 mg per kilogram of body weight, vomiting was reported in about 11% of people, and one severe allergic reaction has been reported (Schwalfenberg, 2021).

Stop taking NAC and get medical help right away if you notice signs of an allergic reaction, such as a rash, hives, swelling of the face or throat, or trouble breathing.

Who Should Not Take NAC Without Checking First

• People on blood thinners or with a bleeding disorder. NAC has anticoagulant and platelet inhibiting properties, so a clinical review cautions against its use in people with bleeding disorders or on blood thinners (Schwalfenberg, 2021). If you have surgery planned, tell your surgeon you take NAC.
• People taking nitroglycerin. NAC taken with nitroglycerin may cause low blood pressure. Ask your pharmacist about any other medicines you take.
• People having cancer treatment. The same review urges caution because, in lung cancer models, NAC may increase the growth of tumor cells, and in mouse studies NAC may interfere with how well one chemotherapy drug works. If you have cancer or are having chemotherapy, talk with your oncology team before taking NAC.
• Pregnancy and breastfeeding. Check with your healthcare provider before taking NAC.
• Anyone taking prescription medication or living with a medical condition. Ask your pharmacist or doctor first.

If you drink, our guide to NAC and alcohol covers what trials found when NAC was taken around a night out.

How Much NAC Is Too Much?

Most oral studies use 600 to 1,800 mg a day, often as 600 mg two or three times a day. In respiratory medicine, the maximum licensed dose for long-term use is 600 mg a day (Calverley et al., 2021).

Daily oral dose	What the research says
600 mg	The maximum licensed long-term medical dose
Up to 1,200 mg	Described as generally well tolerated
600 to 3,000 mg	A review of 41 studies found a safety profile similar to the standard dose
8,000 mg	Did not cause clinically significant reactions in one group of patients; far above typical use

Higher doses are not automatically better, because they can bring more stomach upset. Follow your product label and check with a clinician before taking more than 1,200 mg a day.

Is it safe to take NAC every day?

The 41-study review looked at long-term daily use, and NAC was generally well tolerated. If you plan to take it daily for the long term, it is worth discussing with a healthcare provider, especially if you take other medicines.

NAC Nasal Spray Side Effects

A nasal spray takes a different route: it is absorbed through the lining of the nose instead of being swallowed like a capsule, so some of each dose skips digestion. Sprays also use much smaller doses than capsules. Side effects are usually mild and short-lived:

• Brief stinging or irritation in the nose.
• A runny nose for a few minutes.
• A sulfur smell or taste.

Check with a healthcare provider before using a NAC spray if you have had nasal surgery, and stop using it if you get repeated nosebleeds. Our NAC nasal spray guide compares the spray with capsules, and the NAC Nasal Spray FAQ answers common questions.

Where Our NAC Nasal Spray Fits

Our NAC Nasal Spray gives 50 mg of N-acetylcysteine per serving (one spray in each nostril), dissolved in alkaline water, with 30 servings per bottle. It is made with third-party tested ingredients. It is a low-dose, needle-free way to take NAC, and many people use it alongside capsules rather than instead of them.

If you want glutathione itself rather than its building block, see our Glutathione Nasal Spray. For how NAC compares with another popular ingredient, see NAC vs NMN.

Frequently Asked Questions

Is NAC dangerous?

Not for most healthy adults at the doses used in studies. Reviews describe NAC as generally well tolerated, and the FDA said its initial review had not revealed safety concerns. People on blood thinners or nitroglycerin, and anyone pregnant, breastfeeding or having cancer treatment, should check with a clinician first.

What are the most common side effects of NAC?

Nausea, vomiting, diarrhea, and an unpleasant sulfur smell and taste. Taking NAC with food and splitting larger doses can help.

Can NAC thin your blood?

NAC has anticoagulant and platelet inhibiting properties. If you take a blood thinner, have a bleeding disorder or have surgery planned, talk with your doctor before taking NAC.

Can I take NAC with nitroglycerin?

Not without your doctor's advice. Taken together, they may cause low blood pressure.

How much NAC is too much?

Studies commonly use 600 to 1,800 mg a day, and a review of studies up to 3,000 mg a day found a safety profile similar to the standard 600 mg dose. Follow your label and check with a clinician before taking more than 1,200 mg a day.

Is NAC banned by the FDA?

No. The FDA considers NAC excluded from the dietary supplement definition, but in 2022 it said it would use enforcement discretion for certain NAC products labeled as dietary supplements.

Can NAC make you drowsy?

Drowsiness is listed among the rarer side effects of NAC. If you feel drowsy after taking it, avoid driving and talk with your healthcare provider.

Does NAC nasal spray have side effects?

Usually mild ones: brief stinging, a runny nose or a sulfur smell. It uses a much smaller dose than capsules.

Are NAC side effects common?

No. Most people tolerate NAC well. In a review of 41 studies at 600 to 3,000 mg a day, stomach symptoms were no more common than in the control groups. When side effects do happen, they are usually mild stomach upset or a sulfur smell and taste.

What are the side effects of NAC at 600 mg?

Usually few. In a small study of 30 healthy volunteers taking 600 mg twice a day for three days, two people reported a total of three side effects, including a headache. 600 mg a day is also the maximum licensed long-term medical dose.

What helps if NAC upsets my stomach?

Take it with food, split larger doses (for example 600 mg in the morning and 600 mg in the evening), start low and stay within the label, or try a different form such as effervescent tablets. A nasal spray uses a much smaller dose and is not swallowed.

Sources

• Schwalfenberg GK. N-Acetylcysteine: a review of clinical usefulness (an old drug with new tricks). J Nutr Metab. 2021. PubMed 34221501
• Calverley P, Rogliani P, Papi A. Safety of N-acetylcysteine at high doses in chronic respiratory diseases: a review. Drug Saf. 2021. PubMed 33326056
• Papi A, Di Stefano AFD, Radicioni M. Pharmacokinetics and safety of single and multiple doses of oral N-acetylcysteine in healthy Chinese and Caucasian volunteers: an open-label, phase I clinical study. Adv Ther. 2021. PubMed 33146843
• U.S. Food and Drug Administration. FDA releases final guidance on enforcement discretion for certain NAC products. 2022. fda.gov

These statements have not been evaluated by the Food and Drug Administration. This content is for education and is not medical advice. Our products are not intended to diagnose, treat, cure or prevent any disease. Talk with a healthcare provider before starting a new supplement if you are pregnant, breastfeeding or taking medication.