Plaquenil and Prednisone are two different things.
Plaquenil is a hydroxychloroquine which is an anti-malarial drug. It is generally prescribed for the management of cutaneous manifestations in Lupus (like the malar rash, hair loss, skin lesions, mouth ulcers). It is usually prescribed in about 400 mg per day.
It is sometimes prescribed for polyarthritis in patients with SLE, but generally this is done in conjunction with immunosuppressive drugs and NSAIDs (nonsteroidal anti-inflammatory drugs)
Prednisonse is a Corticosteroid and is one of the most common treatments for SLE. These drugs have the effect of controlling the symptoms of SLE, primarily by the anti-inflammatory action and by inhibiting the production of antibodies. They are usually prescribed in low doses, to be taken on a regular basis and may be taken in higher doses for limited periods of time to control flares of disease activity.
They can have some serious side-effects with long term use and/or high dosages. Some of the side-effects are: weight gain, increased susceptibility to infection and/or masking of symptoms of infection, slowed healing of injuries, easy bruising, unusual hair growth, cataracts, diabetes mellitus, absence of menstruation, coronary artery disease, facial swelling (moon face), elevated blood pressure, mood swings, sleep problems, glaucoma. Not everyone will have all of these side-effects and they will vary with each individual.
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