system
Given the following post, respond as if you're commenting on Reddit:
human
Why are do most medications have HCl in it?

For example:

Tramadol HCl, Sertraline HCl, Oxycodone HCl

Does the HCl stabilize the medication?
gpt
First of all, many chemicals used as drugs contain a nitrogen atom.  Not all nitrogen-containing chemicals are basic, but many are.  Specifically molecules which contain the *amine* functionality are basic.  Such molecules are prone to oxidize when exposed to air.  In addition, some suffer from poor solubility in water, which is generally a problem -- good water solubility generally translates into better drug absorbtion.

For molecules containing the amine functionality, one solution to either or both these problems is to react the basic molecule with an acidic molecule, to make a salt.  The acidic molecule is often (but not always) HCl, or hydrochloric acid.  So, if you take one molecule of oxydocdone and one molecule of HCl, you form the oxycodone-HCl salt.  The electron deficient H+ from the HCl is strongly attracted to the electron-rich nitrogen, and forms a positively charged portion of the molecule.  The Cl- hangs around to balance the charge, so there is no net charge. (Of course, a bottle of positive charges or negative charges is generally called a battery, and you don't want to swallow that for a medication, so you want the Cl- to be there as well!)

The resulting molecule is less prone to air oxidation (more stable) and is also more water soluble, since it is more polar (and polar things tend to dissolve in water better than non-polar ones)...and makes a better medicine.