Drug Interactions between aspirin / calcium carbonate and evening primrose

MONITOR: Chronic administration of antacids may reduce serum salicylate concentrations in patients receiving large doses of aspirin or other salicylates. The mechanism involves reduction in salicylate renal tubular reabsorption due to urinary alkalinization by antacids, resulting in increased renal salicylate clearance. In three children treated with large doses of aspirin for rheumatic fever, serum salicylate levels declined 30% to 70% during coadministration with a magnesium and aluminum hydroxide antacid. Other studies have found similar, albeit less dramatic results. Antacids reportedly have no effect on the oral bioavailability of aspirin in healthy adults. However, administration of antacids containing either aluminum and magnesium hydroxide or calcium carbonate two hours before aspirin dosing led to reduced absorption of aspirin in uremic patients.

MANAGEMENT: Patients treated chronically with antacids (or oral medications that contain antacids such as didanosine buffered tablets or pediatric oral solution) and large doses of salicylates (i.e. 3 g/day or more) should be monitored for potentially diminished or inadequate analgesic and anti-inflammatory effects, and the salicylate dosage adjusted if necessary.

Theoretically, use of borage or evening primrose oil with anticoagulants or antiplatelet aggregation drugs may increase the risk of bleeding in some patients. In one study, 12 hyperlipidemic males took gamma linolenic acid 240 mg and linolenic acid 2200 mg (both main components of borage and evening primrose oil) daily for 4 months. After 4 months on this supplementation and compared to baseline, platelet aggregation decreased by 50% when platelets were aggregated with adenosine diphosphate (ADP), and by 60% with adrenaline. Also, in the same study platelet thromboxane B2 levels were reduced by 54% as compared to placebo. However, another study suggests platelet aggregation in healthy patients may not be affected by borage oil supplementation. Until further information is available, clinical and laboratory observation for hematologic complications is recommended. Patients should be advised to promptly report any signs of bleeding to their physician, including prolonged bleeding from cuts, increased menstrual flow, vaginal bleeding, nosebleeds, bleeding of gums from brushing, unusual bruising, red or brown urine, or red or black stools.