Bolting on a water-loving group
Phase II is the *conjugation* stage. An enzyme takes the chemical handle (an –OH, –NH₂, or –COOH, either native to the drug or freshly added in phase I) and attaches a large, polar molecule to it. The result is usually heavy, very water-soluble, and almost always inactive — well primed for excretion.
The most common route is glucuronidation, which attaches glucuronic acid. Other conjugation pathways add sulfate, acetyl, methyl, or amino-acid groups. Each needs a cofactor — a high-energy donor molecule the body must supply — which is why conjugation can become limited when those supplies run short.
Drug-OH + glucuronic-acid (from UDPGA cofactor)
| UGT enzyme
v
Drug-O-glucuronide (large, polar, inactive)
--> urine or bilePhase I and phase II are partners, not a fixed sequence
The names suggest 'phase I then phase II,' but it is not a rule. Some drugs already carry a usable handle and go straight to conjugation. Others are cleared by phase I alone. And the two phases can run in either order. Think of them as two tools the liver picks from, not a fixed assembly line.
The detour: enterohepatic circulation
Large conjugates are often dumped into bile and sent to the gut through biliary excretion. You might expect them to leave in the stool — but gut bacteria can snip the conjugate apart, freeing the original lipophilic drug, which is then reabsorbed. This loop is enterohepatic circulation.
Enterohepatic circulation recycles the drug and lengthens its stay in the body, which can prolong a drug's effect. It also explains a few real-world quirks: why certain oral contraceptives can fail when antibiotics disturb gut bacteria, and why some poisonings are treated by giving repeated activated charcoal to interrupt the loop.