Rifampicin Bentonite

The serum levels of rifampicin are approximately halved if aminosalicylic acid granules containing bentonite are given. 

In 30 patients with tuberculosis the serum levels of rifampicin 10 mg/kg were reduced by more than 50%, from 6.06 to 2.91 micrograms/mL, at 2 hours by aminosalicylate. Later studies in 6 healthy subjects showed that this interaction was not due to the aminosalicylic acid itself but to the bentonite, which was the main excipient of the granules. The rifampicin AUC was statistically unchanged in the presence of sodium aminosali-... 

Other studies confirm this marked reduction in serum rifampicin levels in the presence of bentonite in aminosalicylic acid granules. ... 

The bentonite excipient in the aminosalicylic acid granules adsorbs the rifampicin onto its surface so that much less is available for absorption, which results in reduced serum levels. Bentonite is a naturally occurring mineral (montmorillonite) consisting largely of hydrate aluminium silicate, and is similar to kaolin. 

Boman G, Lundgren P, Stjemstrom G. Mechanism of the inhibitory effect of PAS granules cn die absorption of rifampicin adsorption of ri mpicin by an exc ienl, bentonite. EurJ Clin Pharmacol (1975) 8,293-9. 

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