Phase II: bolting on a soluble anchor
Once phase I has exposed a reactive handle — usually a hydroxyl, amine, or carboxylic acid — phase II metabolism takes over. This is conjugation: the enzyme transfers a large, highly polar group from an activated cofactor onto the drug. The result is almost always more water-soluble, usually inactive, and primed for excretion. Conjugation is the body's way of writing *finished, please remove* on a molecule.
The most common conjugation is glucuronidation, run by the UGT enzymes, which attach a sugar-acid (glucuronic acid) to give a bulky, charged glucuronide. Other phase II routes include sulfation, acetylation, methylation, and glutathione conjugation — the last one being especially important for mopping up dangerous reactive species, which the next guide covers.
The first-pass gauntlet
Now follow an oral dose. After a pill dissolves, the drug is absorbed across the gut wall — but the blood draining the gut does not go straight to the body. It flows first through the portal vein into the liver. So before a single molecule reaches the heart and general circulation, it must survive two waves of metabolism: enzymes in the gut wall, then the full metabolic might of the liver. This pre-systemic loss is the first-pass effect.
A drug can be perfectly absorbed yet still have terrible oral bioavailability if the first pass destroys most of it. This is why some drugs work as an injection but fail as a tablet, and why a heavily-metabolised molecule may need a much larger oral dose than intravenous dose. High first-pass clearance also makes a drug's blood levels vulnerable to anything that changes liver enzyme activity — the basis of many drug–drug interactions.