Flanax is an over-the-counter pain reliever and fever reducer. Its active ingredient is naproxen sodium (220 mg per tablet), a nonsteroidal anti-inflammatory drug, or NSAID, in the same family as ibuprofen. It’s widely available in the U.S. and Latin America and works by blocking the chemicals your body produces in response to injury or illness that cause pain, swelling, and fever.
Conditions Flanax Treats
Flanax is labeled for temporary relief of a broad range of everyday aches and pains:
- Headaches and toothaches
- Backaches and muscular aches
- Minor arthritis pain
- Menstrual cramps
- Cold symptoms and fever
There’s also a version specifically marketed as Flanax Menstrual Pain Reliever, though the active ingredient and dose are the same. Naproxen sodium is particularly effective for menstrual cramps because it reduces the inflammation-related compounds (called prostaglandins) that cause uterine contractions and cramping.
How Long the Relief Lasts
One of the main advantages of Flanax over other common painkillers is its long duration. A single dose provides pain relief for up to 12 hours, according to FDA labeling. That’s roughly twice as long as ibuprofen, which typically wears off in four to six hours.
A clinical trial on dental pain found that people who took naproxen sodium went significantly longer before needing a second dose compared to those who took ibuprofen. Fewer naproxen-treated patients needed rescue medication over a 24-hour period. This longer window makes Flanax a practical choice when you don’t want to re-dose throughout the day, such as overnight for back pain or during a workday for arthritis flare-ups.
Dosage for Adults
The standard over-the-counter Flanax tablet contains 220 mg of naproxen sodium. For general pain or fever, the typical approach is one tablet every 8 to 12 hours. You can take two tablets for the first dose if needed, then drop to one tablet per dose after that. The maximum for OTC use is generally three tablets (660 mg) in 24 hours unless a doctor directs otherwise.
For prescription-strength naproxen used for joint conditions, doses can go up to 1,000 mg per day. Painful periods are sometimes treated with a higher initial dose of 500 mg followed by 250 mg every 6 to 8 hours. These higher doses require a prescription and medical guidance.
Common Side Effects
Most people tolerate Flanax well at OTC doses for short-term use. The most frequent side effects are mild: upset stomach, nausea, headache, and loss of appetite. Taking the tablet with food or a glass of milk can reduce stomach irritation.
More serious problems are uncommon with occasional use but become more likely the longer you take the drug. The most important ones to know about are stomach bleeding (which can happen without warning symptoms), kidney strain, and increased blood pressure. Signs of stomach bleeding include black or tar-like stools, vomiting material that looks like coffee grounds, or unusual bruising.
Who Should Avoid Flanax
Certain people should not take naproxen sodium at all. If you’ve ever had an allergic reaction to aspirin or ibuprofen, especially one involving hives, facial swelling, or breathing difficulty, naproxen can trigger the same response. People with aspirin-sensitive asthma are at particular risk for severe reactions.
You should also avoid Flanax if you have advanced kidney disease or severe heart failure. The FDA warns against using any NSAID (except aspirin) in the weeks following coronary artery bypass surgery due to elevated cardiovascular risk. Pregnant women should avoid naproxen at 30 weeks of gestation and beyond because it can affect fetal heart development.
Your risk of stomach bleeding is higher if you are over 60, have a history of ulcers, take blood thinners or steroid medications, use other NSAIDs at the same time, or drink three or more alcoholic beverages daily. All NSAIDs except aspirin also carry a warning about increased risk of heart attack and stroke, particularly with long-term use or at higher-than-recommended doses.
How Flanax Compares to Ibuprofen
Flanax and ibuprofen belong to the same drug class and work the same way. The practical difference comes down to dosing convenience. Because naproxen sodium lasts up to 12 hours per dose, you take it less often. Ibuprofen needs to be taken every 4 to 6 hours for continuous relief, which means more doses throughout the day and more chances to miss one.
In terms of raw pain relief, the two drugs are comparable at standard OTC doses. The dental pain study mentioned above found no major difference in how much pain each drug relieved at its peak. The difference was in how long that relief held. If your pain is short-lived (a headache that will pass in a few hours), ibuprofen works fine. For pain that lingers, such as arthritis, back pain, or menstrual cramps, naproxen’s longer action is a meaningful advantage.
One thing to never do is combine Flanax with ibuprofen or any other NSAID. Stacking two drugs from the same class doesn’t improve pain relief but does multiply the risk of stomach bleeding and kidney problems.