Naproxen, sold under the brand name Aleve among others, is a nonsteroidal anti-inflammatory drug (NSAID) used to treat pain, menstrual cramps, and inflammatory diseases such as rheumatoid arthritis, gout and fever.

[9] Common side effects include dizziness, headache, bruising, allergic reactions, heartburn, and stomach pain.

[8] Severe side effects include an increased risk of heart disease, stroke, gastrointestinal bleeding, and stomach ulcers.

[8] As an NSAID, naproxen appears to exert its anti-inflammatory action by reducing the production of inflammatory mediators called prostaglandins.

[1] Naproxen sodium is used as a "bridge therapy" in medication-overuse headache to slowly take patients off other medications.

Extended-release formulations are more useful for the treatment of chronic, or long-lasting, conditions, in which long-term pain relief is desirable.

[17] As with all non-steroidal anti-inflammatory medications (NSAIDs), naproxen use should be avoided in pregnancy due to the importance of prostaglandins in vascular and renal function in the fetus.

[18] Common adverse effects include dizziness, drowsiness, headache, rash, bruising, and gastrointestinal upset.

[21][22] As with other non-COX-2 selective NSAIDs, naproxen can cause gastrointestinal problems, such as heartburn, constipation, diarrhea, ulcers and stomach bleeding.

[1] Naproxen poses an intermediate risk of stomach ulcers compared with ibuprofen, which is low-risk, and indometacin, which is high-risk.

[26][27] COX-2 selective and nonselective NSAIDs have been linked to increases in the number of serious and potentially fatal cardiovascular events, such as myocardial infarctions and strokes.

[29] A study found that high-dose naproxen induced near-complete suppression of platelet thromboxane throughout the dosing interval and appeared not to increase cardiovascular disease (CVD) risk, whereas other non-aspirin high-dose NSAID regimens had only transient effects on platelet COX-1 and were associated with a small but definite vascular hazard.

In the United Kingdom, 250 mg tablets of naproxen were approved for OTC sale under the brand name Feminax Ultra in 2008, for the treatment of primary dysmenorrhoea in women aged 15 to 50.

[48] Naproxen has been found in groundwater and drinking water in concentrations high enough to have adverse effects on invertebrates including fungi, algae, bacteria and fishes.

[49] Naproxen is not thoroughly removed by conventional water treatment methods,[50] and its degradation pathways in the environment are limited.

[53] Although the levels are generally low enough to not be acutely toxic, sub-lethal effects may still occur,[54] such as reduced photosynthetic ability.