K got home so I'll explain this to you:
Amphetamine has a pKa of 9.8. This means that it's a soft base. If the substance is placed in a solution with a pH = 9.8, half of it will be neutral. If however, the pH of the solution is acidic, most of the amphetamine will be protonated, meaning it has a positive charge. For a substance to be absorbed through stomach and intestines it needs to be neutral.
Using Henderson Hasselbalch equation:
Let's say your [A-] is 10^-9M and your [HA] is 0.1M, you have:
pH = 9.8 + log(10^-9/0.1) = 1.8
It means that 10^-9M are neutral and 0.1M protonated. As soon as some of the 10^-9M starts being absorbed, the equilibrium shifts towards the creation of A- because you need to keep the ratio constant. Because 10^-9M is a small amount, the rate of absorption is low.
Now if you take antacids and somehow make your stomach pH = 6, then:
6 = 9.8 + log([A-]/[HA])
log([A-]/[HA]) = -3.8
[A-]/[HA] = 10^-3.8
[A-]/[HA] = 0.000158
[A-]+[HA] = 0.1mol
[HA] = 0.1 - [A-]
substitute:
[A-]/(0.1 - [A-]) = 0.000158
[A-] = 0.000158(0.1 - [A-])
[A-] = 0.0000158 - 0.000158[A-]
[A-] - 0.000158[A-] = 0.0000158
[A-](1-0.000158) = 0.0000158
[A-] = 0.0000158/(1-0.000158)
[A-] = 1.58 x 10^-5M
The concentration of neutral amphetamine is 15,800x greater so imagine what the absorbtion is like.
That is before amphetamine goes in the blood. Once it's in the blood it's a different story. The molecule has to stay neutral to pass the blood brain barrier. That's something else. If your blood is too acidic, some amphetamine never reaches the brain and is excreted, it's probably what you call ''destroyed''.