Nyquil Warnings

Nyquil is an over-the-counter medication used to treat cough, cold and flu symptoms. The product, available as liquid capsule or in liquid form, contains dextromethorphan, acetaminophen, doxylamine and pseudoephedrine as active ingredients. In combination, these ingredients help treat a stuffy or runny nose, cough, pain and fever. Side effects of Nyquil include dry mouth, nausea, insomnia and dizziness. Serious complications may occur when taking Nyquil.

Use with Caution for Children

Misusing cold medications can cause death for children, especially young ones. Consult a physician before giving cold medicines, including Nyquil, to a child. The product may cause increased drowsiness in children.

Do Not Mix with MAO Inhibitors

MAO inhibitors, used to treat clinical depression, can cause serious or life-threatening complications. People taking an MAO inhibitor medication must stop that medicine at least 14 days before using Nyquil, according to Drugs.com 1. Mixing Nyquil with an MAO inhibitor can intensify the effect on the central nervous system and cause increased drowsiness and respiratory depression.

Avoid Products Containing Acetaminophen

Acetaminophen is a drug that helps reduce fevers and relieves pain. Taking other medications containing acetaminophen can cause too much of the drug to accumulate in the body. The maximum daily limit per day for healthy adults is 4g acetaminophen, according to Drugs.com.

Beware of Drug Interactions

Additional medications to avoid while taking Nyquil include beta-blockers that help control blood pressure, seizure medications such as phenytoin and the appetite suppressant sibutramine. Additional medications may cause complications when used with Nyquil. People taking medications, such as blood thinners, may experience an increased risk of bleeding while taking Nyquil. Consult a physician or pharmacist before taking any new medications.

Watch for Reactions to Medical Conditions

Several medical conditions--including glaucoma, liver disease, enlarged prostate and asthma--may worsen after taking Nyquil. People suffering from a medical condition must speak with a physician before taking new medications. Women who are pregnant or breastfeeding must consult a doctor before taking Nyquil to confirm the safety of the baby.

Do Not Overdose

People who take more than the recommended amount of Nyquil must seek immediate medical attention or contact a Poison Control Center, according to Procter & Gamble, the manufacturer of Nyquil. An overdose in the medication can cause serious harm due to the acetaminophen content. Symptoms of an overdose of acetaminophen include:

• confusion
  
• weakness
  
• stomach pain
  
• nausea
  
• vomiting
  
• according to Drugs.com

After liver damage occurs from the overdose, symptoms may include jaundice and dark urine.