Heroin reacts with acids to form salts which are soluble in water.
Consider ammonia gas, which is simply a Nitrogen atom bonded to three Hydrogen atoms: NH
3. In the presence of an acid (or even water, since some H
2O molecules tend to separate into H
+ and OH
- by themselves), it can assimilate a H
+ ion and form an ammonium ion NH
4+.
Now, any group that can form a single covalent bond can substitute for any one, two or even all three of the Hydrogens in ammonia, without prejudice to the Nitrogen's ability to latch onto a H
+. Such molecules, with a substituted-ammonia structure, are called
amines. And any amine which ordinarily does not dissolve in water can be "dissolved" in an acid. (It's not really a simple dissolution; there is actually a chemical reaction going on, of the form
*N + H+ → *NH+
where the * is a wildcard for whatever is attached to the Nitrogen. The positive "aminium" ion will bond with the negative ion from the acid.) The original, unreacted form of the amine is referred to as the
freebase to distinguish it. Also, if a dilute alkali is added to a solution of an amine salt, there will be a chemical reaction of the form
*NH+ + OH- → *N + H2O
and the freebase form will precipitate out. The positive ion from the hydroxide will bond with the negative ion from the amine salt to form a simple salt.
The freebase form of an amine has a lower boiling point than any of its salts, so is most easily vaporised; and in this state it is readily absorbed through the lungs. However, in order to inject heroin directly into the bloodstream, it needs to be reacted with an acid; usually citric acid -- really desperate junkies have even been known to use lemon juice, for want of anything better -- or ascorbic acid. If the freebase form were swallowed, it would be converted by stomach juices -- dilute hydrochloric acid -- into the hydrochloride salt. This probably is the source of the confusion about acids and antacids. The antacid drug used is taken for a secondary effect: it helps the loperamide (which is an amine, and bonds with the opioid receptors) stay on the "brain" side of the blood-brain barrier. It would normally be repelled from there by the action of a substance known as PGP. Co-administering a PGP inhibitor with loperamide is what allows it to stay inside the BBB. But the PGP inhibitor need not be an antacid, and not all antacids are PGP inhibitors. It's just that a certain drug commonly prescribed as a more powerful alternative to over-the-counter antacids happens also to be a PGP inhibitor.