Plaquenil (hydroxychloroquine) is primarily used to treat autoimmune conditions, especially lupus and rheumatoid arthritis. It also has an older but more limited role in preventing and treating malaria. The drug works by calming an overactive immune system, making it one of the most widely prescribed medications in rheumatology.
Lupus
Lupus is the condition most closely associated with Plaquenil. Nearly all patients with systemic lupus erythematosus are started on it, and many stay on it for years or even decades. The drug reduces the frequency and severity of lupus flares, the unpredictable episodes of inflammation that can damage the skin, joints, kidneys, and other organs. It also lowers overall disease mortality.
Dosing matters significantly. Research from Mass General Brigham found that lupus patients taking a lower weight-based dose had a six-fold higher risk of moderate to severe flares compared to those on a higher dose. The same group also found that underdosing was linked to more hospitalizations for flares. The current target is around 5 mg/kg of body weight per day, with evidence suggesting the sweet spot may be slightly above that threshold. Going much higher doesn’t continue to reduce flare risk and raises the chance of side effects.
Rheumatoid Arthritis
In rheumatoid arthritis, Plaquenil belongs to a class of medications called disease-modifying anti-rheumatic drugs. Unlike painkillers that only mask symptoms, it works deeper in the disease process to reduce joint pain and swelling and, over time, helps prevent the permanent joint damage that untreated RA can cause. It’s frequently prescribed alongside other medications in the same class, particularly for early or mild-to-moderate disease.
One important thing to know: Plaquenil is not a fast-acting drug. If you’re starting it for rheumatoid arthritis, symptom improvement typically takes several weeks to months. This slow onset catches some people off guard, but it doesn’t mean the medication isn’t working. Doctors generally ask patients to give it a full trial before deciding whether it’s effective.
Malaria Prevention
Plaquenil was originally developed as an antimalarial drug, and it’s still used for that purpose in limited settings. However, widespread resistance has dramatically narrowed where it works. According to the CDC, hydroxychloroquine is only effective in areas where the related drug chloroquine still works, which at this point means parts of Central America and the Caribbean.
For travelers heading to those regions, the protocol involves taking one dose per week starting at least one week before arrival, continuing weekly throughout the trip, and then for four consecutive weeks after leaving. The weekly adult dose is 400 mg. For children, the dose is calculated by weight but never exceeds the adult amount.
Other Autoimmune Conditions
Beyond its two main indications, doctors prescribe Plaquenil off-label for several other autoimmune and connective tissue diseases. Sjögren’s syndrome, a condition that causes severe dryness of the eyes and mouth along with fatigue and joint pain, is one of the most common. A large randomized trial (the JOQUER study) tested whether hydroxychloroquine improved dryness, pain, and fatigue in Sjögren’s patients over six months. The results were mixed, and the drug is not considered a clear-cut treatment for Sjögren’s, but some rheumatologists still use it when other options are limited.
Plaquenil is also sometimes used for dermatomyositis, mixed connective tissue disease, and certain skin conditions related to autoimmune activity. These uses are based on its general ability to dampen immune overactivity rather than on large dedicated trials.
How It Works in the Body
For years, scientists thought Plaquenil worked by broadly disrupting the internal chemistry of immune cells, essentially raising the pH inside small compartments called lysosomes and gumming up their function. That theory has largely been replaced by a more specific explanation. The drug blocks toll-like receptors, sensors on certain immune cells that detect fragments of DNA and RNA and trigger an inflammatory cascade. In autoimmune diseases, these sensors are activated inappropriately, causing the immune system to attack the body’s own tissues. By binding directly to the nucleic acids that activate those sensors, hydroxychloroquine prevents the chain reaction that produces key inflammatory signals.
This targeted mechanism explains why Plaquenil is effective across multiple autoimmune diseases. It dials down the same misdirected immune signaling that drives lupus flares, joint inflammation in RA, and tissue damage in other conditions.
What Plaquenil Does Not Treat
During 2020, hydroxychloroquine received enormous public attention as a potential COVID-19 treatment. The FDA initially allowed emergency use based on lab studies, not human trials. Within months, clinical trials showed it did not effectively treat COVID-19, did not prevent infection, and caused serious heart problems in some patients at the doses being used. The FDA revoked its emergency authorization in June 2020. The medical consensus is clear: hydroxychloroquine has no role in treating or preventing COVID-19.
Eye Monitoring and Long-Term Safety
The most significant long-term risk of Plaquenil is damage to the retina, a condition called hydroxychloroquine retinopathy. This is rare in the first several years of use, but the risk increases with time. For patients who keep their daily dose at or below 5 mg/kg of actual body weight, the chance of retinal toxicity is below 1% within the first five years and stays under 2% even after ten years.
The American Academy of Ophthalmology recommends a baseline eye exam soon after starting the drug, including specialized imaging of the retina. Annual screening is advised for ongoing use, though it can be deferred during the first five years if there are no additional risk factors. The daily dose ceiling recommended by the FDA is 6.5 mg/kg (or 600 mg total, whichever is lower), because retinal damage rates climb above that threshold. Other factors that increase risk include kidney problems and use of certain other medications.
Early detection through regular eye exams is the key safeguard. Retinal toxicity caught early, before symptoms appear, generally has a better outlook than damage discovered after vision changes have already started.