Nyquil: Its Side Effects, Composition and Period of Effectiveness

woman blowing her nose

Nyquil is one of the most common medicinal substances in the country, used to treat the symptoms of mild cough, cold and allergies. You can probably find a bottle of Nyquil in every medicine cabinet, alongside bottles of expired melatonin and boxes of Band-Aid. It’s an over-the-counter medicine like aspirin or ketoconazole, which means you don’t need a doctor’s prescription to acquire it, which is why it’s so common and easy to acquire.

But what do you know about Nyquil?

Today, learn the answers to common questions about this medication such as what are the side effects of Nyquil, what are its ingredients and how long does Nyquil take to kick in.

What are the Side Effects of Nyquil?

man suffering from a headache
Side effects of Nyquil can include disorientation and dizziness. Photo by Andrea Piacquadio from Pexels

Like most medications, Nyquil can cause some side effects, but they are relatively rare. Serious side effects of Nyquil are even rarer, but they can still occur. It’s important to know what these side effects are, so you can tell a physician should they become serious.

The side effects of Nyquil, from light to serious, include the following conditions.

Mild Side Effects

  • Blurred vision
  • Sudden drowsiness
  • Sudden dizziness
  • Feeling nervous
  • Developing nausea
  • Constipated bowels
  • Dryness of your mouth, throat and or nose.

Serious Side Effects

  • Mental disorientation, like confusion or hallucination
  • Mild to severe shaking
  • Issues when urinating
  • Arrhythmia (increased heartbeat) or bradycardia (slow heartbeat)

You should remember that if a doctor prescribed you or someone you know with Nyquil that they have weighed the benefits of the medication against the most common side effects. However, if you manifest the serious side effects of Nyquil, you should immediately tell a physician. Even some of the milder side effects can be hazardous in the right conditions.

For example, blurred vision and drowsiness while driving can be very dangerous and even fatal. For this reason, you shouldn’t operate heavy machinery after taking a dose of Nyquil.

Some people may even have allergic reactions to Nyquil, which can go undetected until you use it for the first time. If you develop an allergic reaction to it, you should definitely tell your doctor.

What are the Ingredients of Nyquil?

person holding a flask
The ingredients of Nyquil make it very effective for colds and coughs. Photo by Chokniti Khongchum from Pexels

Despite seeming like a simple medicine you keep in the cabinet for colds and in preparation for allergy season, Nyquil has some very complex ingredients. What are these chemical components that offer such immense relief to clogged noses and feverish children everywhere? Take a closer look at the three main active ingredients of Nyquil.


Acetaminophen is a drug that’s usually used for treating mild pain. It’s an active component in medicine for dealing with headaches, backache and body pain from fly or the cold. In higher dosages, it can be used to deal with the pain from osteoarthritis and menstrual cramps. It’s also helpful for reducing the effects of fever.


Dextromethorphan is a drug used to provide temporary relief from the cough brought on by diseases, like common cold and the flu. Unlike stronger antibiotics and drugs, dextromethorphan doesn’t cure the cause of the cough, merely soothes your respiratory system. It belongs to class of drugs that are called antitussives. These drugs suppress coughing by telling the portion of your brain that causes your coughs to slow down.


Doxylamine belongs to the group of medicines known as antihistamines, which help with relieving the symptoms of respiratory allergies, hay fever and colds. Doxylamine works by preventing your body from producing natural compounds like acetylcholine and histamine. This stops you from getting your nose runny, sneezing too much or making your eyes water.


Nyquil also contains about 10 percent alcohol in its composition. This alcohol is important for turning the active ingredients listed above into liquid and letting your body absorb it. With 10 percent concentration of alcohol, it’s about as alcoholic as a bottle of white wine. This is why one of the side effects of Nyquil can seem like the effects of drinking too much.

How Long Does Nyquil Last?

Nyquil is usually very potent, even in small doses. According to the manufacturer, Nyquil can take up to 30 minutes before it starts working. You may be wondering how long a single dose of Nyquil is effective, just like how people wonder how long does Advil last. Nyquil is pretty potent and with its alcohol content, you have to be very careful about dosages. The manufacturers indicate that you shouldn’t take more than 4 doses of the stuff in a given 24-hour period. This means that a single dose is effective and active for up to 6 hours.

However, this limit can also be because of the side effects of Nyquil and its alcohol content, which can be debilitating. If taken without need and in excessive dosages, Nyquil can be addictive, thanks to the alcohol content. In fact, there are people who have to get treatment for Nyquil addiction and have to learn how to cope with drug abuse.

Nyquil is one of the most common medicines out there, which makes it more important for you to learn more about it. Understanding what are the side effects of Nyquil and what it contains can help you make informed medical decisions and stay safe.

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Medical Disclaimer

The information provided on this website is for general informational purposes only and should not be considered medical advice. The content on the website is not intended to be a substitute for professional medical diagnosis, treatment, or advice. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.


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