The antacid power of five natural clays is reported and compared to two pharmaceutical clay based drugs. The improvement of the antacid properties is attempted through amorphization and calcium carbonate addition. Two mechanisms of pH control are proposed: acido-basic neutralization at amphoteric edge functions and cationic exchange. The later is mostly active for 2:1 clays in the absence of carbonate which induces noticeable acido-basic neutralization. Natural 2:1 clays exhibit high antiacid effect whereas 1:1 clays induce good buffering effect. The antacid effect of the thermally treated 1:1 clays is increased to values (0.20 ≤ ΔpH ≤ 0.41) close to those of the pharmaceutical clay based drug smecta (ΔpH = 0.52). The addition of calcium carbonate to 1:1 clays material also improves their antacid effect. In calcium carbonate/clay mixture, the presence of clay is favorable to a buffering control and a calcium carbonate dosage of interest is observed at 4.5%.