[7][8] Hydroxychloroquine has been studied for an ability to prevent and treat coronavirus disease 2019 (COVID-19), but clinical trials found it ineffective for this purpose and a possible risk of dangerous side effects.

[9] Among studies that deemed hydroxychloroquine intake to cause harmful side effects, a publication by The Lancet was retracted due to data flaws.

[11] Hydroxychloroquine treats rheumatic disorders such as systemic lupus erythematosus, rheumatoid arthritis, and porphyria cutanea tarda, and certain infections such as Q fever and certain types of malaria.

[3] Serious reported neuropsychiatric adverse effects of hydroxychloroquine use include agitation, mania, difficulty sleeping, hallucinations, psychosis, catatonia, paranoia, depression, and suicidal thoughts.

[11] For short-term treatment of acute malaria, adverse effects can include abdominal cramps, diarrhea, heart problems, reduced appetite, headache, nausea and vomiting.

[3] Other adverse effects noted with short-term use of Hydroxychloroquine include low blood sugar and QT interval prolongation.

[11] For prolonged treatment of lupus or rheumatoid arthritis, adverse effects include the acute symptoms, plus altered eye pigmentation, acne, anemia, bleaching of hair, blisters in mouth and eyes, blood disorders, cardiomyopathy,[19] convulsions, vision difficulties, diminished reflexes, emotional changes, excessive coloring of the skin, hearing loss, hives, itching, liver problems or liver failure, loss of hair, muscle paralysis, weakness or atrophy, nightmares, psoriasis, reading difficulties, tinnitus, skin inflammation and scaling, skin rash, vertigo, weight loss, and occasionally urinary incontinence.

Advanced retinopathy is characterized by reduction of visual acuity and a "bull's eye" macular lesion which is absent in early involvement.

[25] These may include sleepiness, vision changes, seizures, coma, stopping of breathing, and heart problems such as ventricular fibrillation and low blood pressure.

[25] Hydroxychloroquine may be quantified in plasma or serum to confirm a diagnosis of poisoning in hospitalized victims or in whole blood to assist in a forensic investigation of a case of sudden or unexpected death.

Plasma or serum concentrations are usually in a range of 0.1-1.6 mg/L during therapy and 6–20 mg/L in cases of clinical intoxication, while blood levels of 20–100 mg/L have been observed in deaths due to acute overdosage.

[19] Care should be taken if combined with medication altering liver function as well as aurothioglucose (Solganal), cimetidine (Tagamet) or digoxin (Lanoxin).

[36] Higher pH within lysosomes causes decreased intracellular processing, glycosylation and secretion of proteins with many immunologic and nonimmunologic consequences.

[37] These effects are believed to be the cause of a decreased immune cell functioning such as chemotaxis, phagocytosis and superoxide production by neutrophils.

[38] Hydroxychloroquine is a weak diprotic base that can pass through the lipid cell membrane and preferentially concentrate in acidic cytoplasmic vesicles.

TLRs are cellular receptors for microbial products that induce inflammatory responses through activation of the innate immune system.

The most accepted model is based on hydrochloroquinine and involves the inhibition of hemozoin biocrystallization, which facilitates the aggregation of cytotoxic heme.

[3] Brand names of hydroxychloroquine include Plaquenil, Hydroquin, Axemal (in India), Dolquine, Quensyl, and Quinoric.

[53][54][55][56][57][58] Administration of chloroquine or hydroxychloroquine to COVID-19 patients, either as monotherapies or in conjunction with azithromycin, has been associated with deleterious outcomes, such as QT prolongation.

[62] Several countries initially used chloroquine or hydroxychloroquine for treatment of persons hospitalized with COVID-19 (as of March 2020), though the drug was not formally approved through clinical trials.

[66] On 24 April 2020, citing the risk of "serious heart rhythm problems", the FDA posted a caution against using the drug for COVID-19 "outside of the hospital setting or a clinical trial".

[68][69] On 15 June 2020, the FDA revoked its emergency use authorization, stating that it was "no longer reasonable to believe" that the drug was effective against COVID-19 or that its benefits outweighed "known and potential risks".

[70][71][72] In fall of 2020, the National Institutes of Health issued treatment guidelines recommending against the use of hydroxychloroquine for COVID-19 except as part of a clinical trial.