DIFLUNISAL — diflunisal tablet
Zydus Pharmaceuticals (USA) Inc.


Diflunisal Tablets, USP

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Cardiovascular Risk

• Nonsteroidal anti-inflammatory drugs (NSAIDs) cause an increased risk of serious cardiovascular thrombotic events, including myocardial infarction and stroke, which can be fatal. This risk may occur early in treatment and may increase with duration of use (see WARNINGS and PRECAUTIONS).

• Diflunisal tablets are contraindicated in the setting of coronary artery bypass graft (CABG) surgery (see CONTRAINDICATIONS and WARNINGS).

Gastrointestinal Risk

• NSAIDs cause an increased risk of serious gastrointestinal adverse events including bleeding, ulceration, and perforation of the stomach or intestines, which can be fatal. These events can occur at any time during use and without warning symptoms. Elderly patients are at greater risk for serious gastrointestinal events (see WARNINGS).


Diflunisal is 2′,4′-difluoro-4-hydroxy-3-biphenylcarboxylic acid. Its structural formula is:


C13 H8 F2 O3 M.W. 250.20

Diflunisal is a stable, white to off-white, crystalline compound with a melting point of 211° to 213°C. It is practically insoluble in hexane and water. It is soluble in most organic solvents including acetone and ethyl acetate and it is slightly soluble in chloroform, carbon tetrachloride and methylene chloride.

Each diflunisal tablet intended for oral administration contains 500 mg of diflunisal. In addition, each tablet contains the following inactive ingredients: colloidal silicon dioxide, croscarmellose sodium, hypromellose, microcrystalline cellulose, pregelatinised starch and sodium stearyl fumarate. Additionally each diflunisal tablets contain opadry blue 03B505010 which contains FD&C blue #2, FD&C yellow #6, hypromellose, polyethylene glycol and titanium dioxide.



Diflunisal is a non-steroidal drug with analgesic, anti-inflammatory and antipyretic properties. It is a peripherally-acting non-narcotic analgesic drug. Habituation, tolerance, and addiction have not been reported.

Diflunisal is a difluorophenyl derivative of salicylic acid. Chemically, diflunisal differs from aspirin (acetylsalicylic acid) in two respects. The first of these two is the presence of a difluorophenyl substituent at carbon 1. The second difference is the removal of the O -acetyl group from the carbon 4 position. Diflunisal is not metabolized to salicylic acid, and the fluorine atoms are not displaced from the difluorophenyl ring structure.

The precise mechanism of the analgesic and anti-inflammatory actions of diflunisal is not known. Diflunisal is a prostaglandin synthetase inhibitor. In animals, prostaglandins sensitize afferent nerves and potentiate the action of bradykinin in inducing pain. Since prostaglandins are known to be among the mediators of pain and inflammation, the mode of action of diflunisal may be due to a decrease of prostaglandins in peripheral tissues.

Pharmacokinetics and Metabolism

Diflunisal is rapidly and completely absorbed following oral administration with peak plasma concentrations occurring between 2 to 3 hours. The drug is excreted in the urine as two soluble glucuronide conjugates accounting for about 90% of the administered dose. Little or no diflunisal is excreted in the feces. Diflunisal appears in human milk in concentrations of 2 to 7% of those in plasma. More than 99% of diflunisal in plasma is bound to proteins.

As is the case with salicylic acid, concentration-dependent pharmacokinetics prevail when diflunisal is administered; a doubling of dosage produces a greater than doubling of drug accumulation. The effect becomes more apparent with repetitive doses. Following single doses, peak plasma concentrations of 41 ± 11 mcg/mL (mean ± S.D.) were observed following 250 mg doses, 87 ± 17 mcg/mL were observed following 500 mg and 124 ± 11 mcg/mL following single 1000 mg doses. However, following administration of 250 mg b.i.d., a mean peak level of 56 ± 14 mcg/mL was observed on day 8, while the mean peak level after 500 mg b.i.d. for 11 days was 190 ± 33 mcg/mL. In contrast to salicylic acid which has a plasma half-life of 2 1/2 hours, the plasma half-life of diflunisal is 3 to 4 times longer (8 to 12 hours), because of a difluorophenyl substituent at carbon 1. Because of its long half-life and nonlinear pharmacokinetics, several days are required for diflunisal plasma levels to reach steady state following multiple doses. For this reason, an initial loading dose is necessary to shorten the time to reach steady-state levels, and 2 to 3 days of observation are necessary for evaluating changes in treatment regimens if a loading dose is not used.

Studies in baboons to determine passage across the blood-brain barrier have shown that only small quantities of diflunisal, under normal or acidotic conditions are transported into the cerebrospinal fluid (CSF). The ratio of blood/CSF concentrations after intravenous doses of 50 mg/kg or oral doses of 100 mg/kg of diflunisal was 100:1. In contrast, oral doses of 500 mg/kg of aspirin resulted in a blood/CSF ratio of 5:1.

Mild to Moderate Pain

Diflunisal is a peripherally-acting analgesic agent with a long duration of action. Diflunisal produces significant analgesia within 1 hour and maximum analgesia within 2 to 3 hours.

Consistent with its long half-life, clinical effects of diflunisal mirror its pharmacokinetic behavior, which is the basis for recommending a loading dose when instituting therapy. Patients treated with diflunisal, on the first dose, tend to have a slower onset of pain relief when compared with drugs achieving comparable peak effects. However, diflunisal produces longer lasting responses than the comparative agents.

Comparative single dose clinical studies have established the analgesic efficacy of diflunisal at various dose levels relative to other analgesics. Analgesic effect measurements were derived from hourly evaluations by patients during eight and twelve hour postdosing observation periods. The following information may serve as a guide for prescribing diflunisal.

Diflunisal 500 mg was comparable in analgesic efficacy to aspirin 650 mg, acetaminophen 600 mg or 650 mg, and acetaminophen 650 mg with propoxyphene napsylate 100 mg. Patients treated with diflunisal had longer lasting responses than the patients treated with the comparative analgesics.

Diflunisal 1000 mg was comparable in analgesic efficacy to acetaminophen 600 mg with codeine 60 mg. Patients treated with diflunisal had longer lasting responses than the patients who received acetaminophen with codeine.

A loading dose of 1000 mg provides faster onset of pain relief, shorter time to peak analgesic effect, and greater peak analgesic effect than an initial 500 mg dose.

In contrast to the comparative analgesics, a significantly greater proportion of patients treated with diflunisal did not remedicate and continued to have a good analgesic effect eight to twelve hours after dosing. Seventy-five percent (75%) of patients treated with diflunisal continued to have a good analgesic response at four hours. When patients having a good analgesic response at four hours were followed, 78% of these patients continued to have a good analgesic response at eight hours and 64% at twelve hours.

Chronic Anti-Inflammatory Therapy in Osteoarthritis and Rheumatoid Arthritis

In the controlled, doubleblind clinical trials in which diflunisal (500 mg to 1000 mg a day) was compared with anti-inflammatory doses of aspirin (2 to 4 grams a day), patients treated with diflunisal had a significantly lower incidence of tinnitus and of adverse effects involving the gastrointestinal system than patients treated with aspirin (see also Effect on Fecal Blood Loss).


The effectiveness of diflunisal for the treatment of osteoarthritis was studied in patients with osteoarthritis of the hip and/or knee. The activity of diflunisal was demonstrated by clinical improvement in the signs and symptoms of disease activity.

In a doubleblind multicenter study of 12 weeks’ duration in which dosages were adjusted according to patient response, diflunisal 500 or 750 mg daily was shown to be comparable in effectiveness to aspirin 2000 or 3000 mg daily. In open-label extensions of this study to 24 or 48 weeks, diflunisal continued to show similar effectiveness and generally was well tolerated.

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