Advil is a nonsteroidal anti-inflammatory drug (NSAID) that is used to relieve pain and reduce fever. According to Drug Patent Watch [1], the active ingredient in Advil is ibuprofen. Ibuprofen works by reducing the production of prostaglandins, which are substances that cause pain, inflammation, and fever. However, one of the common side effects of NSAIDs like Advil is gastrointestinal problems, including acid reflux.

Acid reflux, also known as gastroesophageal reflux disease (GERD), is a condition in which stomach acid flows back into the esophagus, causing a burning sensation in the chest or throat. According to the Mayo Clinic [2], certain medications can trigger or worsen acid reflux symptoms, including NSAIDs like Advil. This is because NSAIDs can irritate the lining of the stomach and esophagus, causing inflammation and increasing the risk of acid reflux.

MedlinePlus [3] recommends that patients who are taking NSAIDs like Advil should take them with food or milk to reduce the risk of stomach upset and acid reflux. Patients who have a history of GERD or other gastrointestinal problems should talk to their doctor before taking NSAIDs, as they may need to take a different type of pain reliever.

In conclusion, Advil contains ibuprofen, which is an NSAID that can cause gastrointestinal problems, including acid reflux. Patients who are taking Advil should take it with food or milk to reduce the risk of stomach upset and acid reflux. Patients who have a history of GERD or other gastrointestinal problems should talk to their doctor before taking Advil or any other NSAID.

Sources:
[1] https://www.drugpatentwatch.com/p/tradename/ADVIL
[2] https://www.mayoclinic.org/diseases-conditions/gerd/expert-answers/heartburn-gerd/faq-20058535
[3] https://medlineplus.gov/ency/patientinstructions/000197.htm