Last reviewed by Editorial Team on January 11th, 2016.
Tylenol and NyQuil basic facts
Tylenol is a brand name for a drug that contains acetaminophen (also known as paracetamol) that has antypiretic and analgesic properties. It is used for reducing pain and fever and also for relieving the symptoms of common cold, cough, headache, toothache, allergies and influenza. Usual per oral doses for Tylenol are 325-650 mg. It is available in tablets, caplets and liquid dosage forms.
Tylenol products on the market are: Tylenol Extra Strength Caplets, Tylenol Regular Strength Tablets, Tylenol 8 HR Extended-Release Caplets, Tylenol 8 HR Arthritis Pain, Tylenol Sinus Congestion & Pain Caplets, Tylenol Cold Multi-Symptom Caplets and Liquid, Tylenol Cold Head Congestion Severe Caplets, Tylenol PM Caplets and Tylenol Cold Sore Throat Liquid. 
NyQuil is a brand name for OTC medication which is used for relieving symptoms of common cold. Product is a combination of 3 active substances: acetaminophen (usually in doses from 325mg to 600mg) used as a pain reliever and fever reducer, dextromethorphan (usually in doses of 15mg/pill or 30mg/30ml) used as a cough suppressant, it also has sedative properties and doxylamine succinate (usually in doses of 6.25mg/pill or 12.5mg/30ml) which is antihistamine with hypnotic effects that is used in combination with other drugs for relieving symptoms of cold and night-time allergy. Some Nyquil’s preparations can contain other active substances such as: NyQuil D that contains nasal decongestive pseudoephedrine (30 mg/15 mL), NyQuil Sinus contains phenylephrine as nasal decongestant, Children’s NyQuil contains chlorpheniramine as antihistamine and has no alcohol.
NyQuil products on the market are: NyQuil Cold/Flu Multisymptom Relief, NyQuil D, Children’s NyQuil, NyQuil Cough, NyQuil Sinus and ZzzQuil. Nyquil is available in syrup and and LiquiCap dosage forms. NyQuil contains sedating antihistamines, hypnotics, or alcohol, and that’s why it should be taken before sleep. 
Mechanisms of action – How Tylenol and NyQuil work in the body
Acetaminophen is highly selective COX-2 inhibitor that inhibits prostaglandin synthesis. It can inhibit COX-2 in CNS. Acetaminophen also acts on the hypothalamic heat regulating centers to produce antipyresis. Acetaminophen’s reactive metabolite N-acetyl-p-benzoquinone imine (NAPQI) can cause a potentially fatal, hepatic necrosis through the process of lipid peroxidation if acetaminophen is overdosed (more than 4g daily). 
Dextromethorphan acts on cough center in medulla by decreasing sensitivity of cough receptors and by interrupting impulse transmission. 
Doxylamine succinate is competitor for H1-receptor sites on target cells and also has anticholinergic effects, lowering labyrinthine function, blocking chemoreceptor trigger zone, and diminishing vestibular stimulation. 
Pseudoephedrine and phenylephrine are sympathomimetics with direct action on the adrenergic receptor system. The vasoconstriction is produced after α-adrenergic receptors activation. 
Chlorpheniramine is antagonist of histamine H1 receptor (HRH1). It has been also shown that this drug is also a serotonin-norepinephrine reuptake inhibitor. 
Can patients mix Tylenol and NyQuil together
Patients should never take Tylenol and Nyquil together! Both products contain acetaminophen as an active ingredient, so if they mix them, they may exceed the allowable daily dose (more than 4g), causing potentially fatal condition called Acetaminophen’s induced hepatic necrosis.
Typical symptoms of acetaminophen induced hepatotoxity are: anorexia, nausea, vomiting and malaise (usually 0.5-24h after ingestion), abdominal pain, anorexia, nausea, vomiting, tachycardia and hypotension (usually 18-72h after ingestion), continued nausea and vomiting, abdominal pain, and a tender hepatic edge, coagulopathy, hypoglycemia and hepatic encephalopathy (usually 72-96h after ingestion). In some critically ill patients, acute renal failure can develop. Multiorgan failure may lead to death.  
So, if patients need to get better analgesic or antipyretic effect and if patient already uses NyQuil for the treatment of cough and cold symptoms, it is always better to take one dose of ibuprofen (Advil, Motrin), naproxen (Aleve, Anaprox) or aspirin together with Nyquil than Tylenol, or any other product containing acetaminophen. However, patients should always talk with their doctors first before taking any drug on their own.
The table below shows side effects after NyQuil and Robitussin administration in recommendable doses. This Incidence can be significantly increased if these drugs are overdosed or taken together.  
|Frequency Not Defined||Nausea, Vomiting, Abdominal Pain, Malaise, Anorexia, Headache,
Hypertension, Tachycardia, Hypotension, Reflex tachycardia, Severe peripheral and visceral vasoconstriction, Tremor, Dizziness, Excitability, Sedation, Sleepiness, Restlessness, Dermatologic rash, Increased appetite, Xerostomia, Anemia blood dyscrasias (neutropenia, pancytopenia, leukopenia), Bilirubin and alkaline phosphatase may increase, Urinary retention
Special precautions and warnings during Tylenol and NyQuil administration:
- Patients shouldn’t take NyQuil if they have used MAO inhibitors drugs such as furazolidone (Furoxone), phenelzine (Nardil), isocarboxazid (Marplan), selegiline (Eldepryl, Emsam, Zelapar), rasagiline (Azilect), or tranylcypromine (Parnate) in the last 14 days. Drug interactions can occur, leading to serious side effects.
- Pregnant patients shouldn’t use NyQuil without medical advice; it is not known whether chlorpheniramine, pseudoephedrine or dextromethorphan will harm an unborn baby. Chlorpheniramine, dextromethorphan, and pseudoephedrine may pass into breast milk and may harm a nursing baby. NyQuil and Tylenol should not be used if mother is breast-feeding a baby.
- Patients with phenylketonuria should avoid these medicines.
- Children younger than 4 years should also avoid NyQuil. This medicine shouldn’t be taken for longer than 7 days in a row.
- Nyquil may impair your thinking or reactions and cause blurred vision. Patients should be careful if they drive or do anything that requires being alert and able to see clearly.
- Patients should avoid alcohol while using these medications. Drinking alcohol can increase certain side effects of these medications. Alcohol may increase the risk of liver damage while taking Tylenol.
- Tylenol can cause unusual results with certain laboratory tests for glucose in the urine.
- Patients should avoid becoming dehydrated or overheated during exercises. Chlorpheniramine can decrease sweating and patients could become more prone to heat stroke
- Patients should avoid these medications if they also take caffeine pills, diet pills, or other stimulants. Taking decongestants (pseudoefedrine, phenylephrine) together with stimulants can increase the risk of unpleasant side effects.
- Medicines such as sedatives, sleeping pills, narcotic pain medicines, medicine for seizures, anxiety or depression, and muscle relaxers can add sedation and sleepiness that can be also caused by dextromethorphan, chlorpheniramine, promethazine, doxylamine and pseudoefedrine.    
Informations retrieved from:
- Can you take NyQuil and Robitussin together
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- Can you take Benadryl and Tylenol together
- Can you take Ketorolac and Tylenol together
- Can I take expired NyQuil products