upon eating extract, transient stomach pain and feeling of acidity or burning ensues... anyway to decrease that without affecting bioavailability? if one takes an antiacid with it, would it decrease absorption?

How OK are you with putting drugs up your butt?

An antacid will actually INCREASE absorption.

any idea how much a dose should be reduced if taken with an antacid?

How are you taking it ? Are you just eating the powder or dissolving it? Capping it may help a lot, but can increase the time it takes for effects to begin.

Buffering it with something is always a good idea. I recommend bombing it (rolling paper wraps) after food. I like the slow release (but not too slow) tea bag effect it has. Spreading out the dose in the gut helps a lot with irritation. Some good fuel food, then fruit (just because) then swallow a few wraps/bombs/watcha-call-em's with a drink. This all creates a diluted timed release with a bit of buffer. With digestion already under way it helps a lot, and speeds things up a bit too.

Is there something left in your extract that might be causing this? Maybe it needs to be washed further to get rid of it.

frankly i cant say ive ever experienced this before, but i can say that after i take mescaline even in low doses it makes me have to sit pretty still through the digestion faze because i feel nauseated. considering taking a tablet of dramamine with my mesc in the future people claim that mixing a crushed tablet with the mescaline itself greatly reduces discomfort.

if your extract isnt snow white or very very light tan you should purify it more.

The hcl crystals are effectively not an acid in your stomach. There is plenty of hcl there anyways and the hcl of your crystals is chemically not available.

Either you have a problem with your extraction and it is impure or you have an other problem with your stomach.

As DrSeltsam said.

HCl is a gas and if your mescaline powder is dry, all the HCl has escaped from it. The only HCl left is bound to the mescaline and not making for its acidity.

I made and ingested 350 mg of mescaline sulfate, with trace amount of sulfuric acid in it (most removed by acetone washes) and it was perfectly okay on stomach.

Cactus and its extracts are known to cause nausea but never heard of acidity. Depends on how pure your extract is....