Hydroxychloroquine sulfate is an antimalarial agent that also serves as a disease-modifying drug for certain autoimmune conditions. In Hong Kong it is available by prescription only in tablet form, most commonly in 200 mg and 400 mg strengths. The medication is marketed under various brand names, such as Plaquenil, but the active ingredient is hydroxychloroquine sulfate.
Hydrochloroquine is taken up into cells and accumulates in acidic organelles such as lysosomes. By raising the pH inside these compartments, it interferes with antigen processing and presentation, which dampens the overactive immune response seen in diseases like systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). The drug also inhibits Toll-like receptor signaling and reduces production of inflammatory cytokines (e.g., interferon-α).
In the malaria parasite, the same pH-alkalinising effect disrupts the parasite’s ability to digest hemoglobin, leading to its death. The onset of action for autoimmune indications typically requires several weeks of therapy, whereas antimalarial effects can be observed within days of loading.
Approved medical uses in Hong Kong (as listed by the Hong Kong Department of Health):
Hydroxychloroquine is indicated for adult patients; pediatric use is reserved for specific circumstances and must be guided by a specialist.
Hydroxychloroquine was investigated early in the COVID-19 pandemic as a potential antiviral agent. Large, well-designed randomized controlled trials, including the RECOVERY trial (UK) and WHO Solidarity trial, demonstrated no clinically meaningful benefit for preventing severe disease or reducing mortality. Consequently, regulatory agencies such as the U.S. FDA and the European Medicines Agency have withdrawn any emergency use authorisation, and the Hong Kong Department of Health does not endorse hydroxychloroquine for COVID-19.
Disclaimer: Use of hydroxychloroquine for COVID-19 is not approved by any regulatory authority. Off-label use requires direct supervision by a qualified healthcare professional, with careful assessment of potential risks and benefits.
Patients with these conditions should discuss alternatives with their prescriber.
These effects usually resolve with continued use or by taking the tablet with food.
Patients experiencing sudden visual changes, palpitations, or severe hypoglycemia should seek immediate medical attention.
Patients should provide a complete medication list, including over-the-counter products and herbal supplements, before starting hydroxychloroquine.
Standard dosing (based on the approved indication):
Malaria prophylaxis: 400 mg once weekly, preceded by a loading dose of 800 mg (two 400 mg tablets) one day before travel, then 400 mg weekly during exposure.
Malaria treatment: 800 mg initially (split as two 400 mg tablets), followed by 400 mg at 6 h, 24 h, and 48 h after the first dose.
Systemic lupus erythematosus / rheumatoid arthritis: 200-400 mg daily, adjusted to the lowest effective dose; some clinicians start at 200 mg and titrate up based on response and tolerability.
Special populations
Renal impairment: Reduce dose proportionally; for creatinine clearance < 30 mL/min, a common approach is 200 mg every other day, but individualisation is essential.
Hepatic impairment: No formal dosage reduction is required for mild disease; severe liver dysfunction warrants cautious use with close monitoring.
Elderly: Start at the lower end of the dosing range (200 mg) and assess tolerability.
Administration - swallow tablets whole with a full glass of water; do not crush or chew.
Missed dose - take the missed tablet as soon as remembered unless the next scheduled dose is within a few hours; do not double the dose.
Overdose - symptoms may include vomiting, cardiac arrhythmia, and seizures. Seek emergency care; supportive measures and cardiac monitoring are the mainstays of treatment.
Discontinuation - abrupt cessation is generally safe, but for long-term autoimmune therapy, physicians may taper the dose to minimise disease flare.
Regular follow-up visits allow clinicians to assess disease control, monitor for toxicity, and adjust dosing as needed.
This article provides educational information about hydroxychloroquine and is not a substitute for professional medical advice. Treatment decisions, including use for unapproved indications, must be made under the guidance of a qualified healthcare provider. The content is intended for informational purposes and does not constitute medical recommendations. Always consult a physician before starting, stopping, or changing any medication regimen.
Hydroxychloroquine does not have a known interaction with vitamin D. However, patients should inform their healthcare provider of all supplements they use to ensure comprehensive care.
The drug should be started one day before entering a malaria-risk area, taken weekly during exposure, and continued for four weeks after leaving the endemic region. Adherence to the schedule is crucial for effectiveness.
Hydroxychloroquine contains an additional hydroxyl group, which reduces its toxicity profile, particularly in the retina, making it the preferred option for long-term autoimmune therapy.
Some evidence suggests hydroxychloroquine may exacerbate psoriasis. Patients with a history of psoriasis should discuss alternative treatments with their dermatologist.
New or worsening blurred vision, loss of central visual acuity, difficulty reading, or seeing “dark spots” may indicate early retinal toxicity and warrant immediate eye-specialist evaluation.
While alcohol does not directly interact with the drug, excessive drinking can impair liver function, potentially affecting hydroxychloroquine metabolism and increasing the risk of toxicity.
Yes, hydroxychloroquine is classified as a prescription-only medication in Hong Kong. A qualified physician must issue a written prescription before a pharmacy can dispense it.
Clinical improvement in lupus often becomes apparent after 4-6 weeks of consistent therapy, though some patients may notice changes earlier. Full therapeutic effect may require several months.
No significant pharmacokinetic interaction has been identified between hydroxychloroquine and combined oral contraceptives. Patients can continue their contraceptive method while on hydroxychloroquine.
Unused tablets should be returned to a pharmacy participating in a medication-take-back program or disposed of according to the Hong Kong Environmental Protection Department’s guidelines for hazardous waste.
