The absorption of rifampicin can be reduced up to about one-third by antacids, but clinical importance of this is uncertain.
When 5 healthy subjects took a single 600mg dose of rifampicin with various antacids absorption of rifampicin was reduced. The antacids caused a fall in urinary excretion of rifampicin as follows: 15 or 30 mL of aluminium hydroxide gel 29 to 31%; 2 or 4 g of magnesium trisilicate 31 to 36%; and 2 g of sodium bicarbonate 21 % (See reference number 1)
Three groups of 15 patients with tuberculosis were given a single oral dose of rifampicin 10 to 12 mg/kg,isoniazid 300mg and ethambutol 20 mg/kg either alone or with about 20 mL of antacid. A significant number of patients had peak rifampicin concentrations below
6.5 micrograms/mL (serum level quoted as necessary to achieve adequate lung concentrations) in group receiving Aludrox (aluminium hydroxide), but no significant effect was noted in group receiving Gelusil (aluminium hydroxide plus magnesium trisilicate) (See reference number 2). However,in a further study in 14 healthy subjects, 30 mL of Mylanta (aluminium/magnesium hydroxide) given 9 hrs before, with and after rifampicin had no effect on rifampicin pharmacokinetics (See reference number 3).
It has been suggested that rise in stomach pH caused by these antacids reduces dissolution of rifampicin and thereby inhibits its absorption. In addition,aluminium ions may form less soluble chelates with rifampicin, and magnesium trisilicate can adsorb rifampicin, both of which would also be expected to reduce bioavailability (See reference number 1).
Direct information seems to be limited to these reports. The effects of 20 to 35 % reductions in rifampicin absorption do not appear to have been assessed,but if antacids are given it would be prudent to be alert for any evidence that treatment is less effective than expected. The US manufacturers of rifampicin advise giving rifampicin 1 hour before antacids (See reference number 4).
Khalil SAH,El-Khordagui LK, El-Gholmy ZA. Effect of antacids on oral absorption of rifampicin. Int J Pharmaceutics (1984) 20, 99–106.
Gupta PR,Mehta YR, Gupta ML, Sharma TN, Jain D, Gupta RB. Rifampicin-aluminium antacid interaction. J Assoc Physicians India (1988) 36, 363–4.
Peloquin CA,Namdar R, Singleton MD, Nix DE. Pharmacokinetics of rifampin under fastingconditions, with food, and with antacids. Chest (1999) 115, 12–18.
Rifadin (Rifampicin). Sanofi-Aventis US LLC. US Prescribing information,March 2007.