# Why is the combination of hydrated aluminium oxide and magnesium hydroxide used to treat heartburn?

Why is the combination of hydrated aluminium oxide and magnesium hydroxide used to treat heartburn?

I get why $\ce{Mg(OH)2}$ would be used since its a strong base and is badly soluble in water. Thus it won't do harm to the oesophagus in comparison to something like $\ce{NaOH}$. Aluminium oxide is also poorly soluble, however it is a weak base so what is the point of combining this substance with magnesium hydroxide instead of just increasing the dose of magnesium hydroxide?

• Different antacids (used to treat heartburn) have different chemical constitutions - see this table on Wikipedia. As to why someone would choose one over the other, I admittedly have no idea – orthocresol Oct 1 '15 at 11:38

Hydrated $\ce{Mg(OH)2}$, aka milk of magnesia, is an osmotic (saline) laxative, as well as antacid.
$\ce{Al(OH)3}$ is also an antacid, but has the opposite effect on the bowels: it's costive, i.e. a binding agent. Aluminum has also been linked (with much uncertainty!) with Alzheimer's disease, so it has become less popular as an antacid.
The combined effects of $\ce{Mg(OH)2}$ as laxative and $\ce{Al(OH)3}$ as a costive might tend to counteract each other, so as to avoid interfering with colonic function.
However, it would be far simpler to use $\ce{CaCO3}$ as an antacid, unless the release of $\ce{CO2}$ bubbles or excess calcium in the diet is an issue.