A drug is a stranger the body wants to evict
Your body did not evolve a welcome mat for the molecules a chemist designs. To the liver, a drug is just another foreign compound — a xenobiotic — to be tagged, taken apart, and shown the door. The machinery that does this is called drug metabolism, and understanding it is one of the most practical skills in medicinal chemistry. If you do not control metabolism, metabolism controls your drug.
The central problem is a contradiction. To cross membranes and reach a target, a drug usually needs to be fairly fatty — high lipophilicity, a comfortable logP. But the body excretes molecules mainly through the kidney into urine, and urine is water. A greasy molecule that the kidney filters out is simply reabsorbed back across the tubule wall. So the body's solution is elegant: chemically rework the drug to make it more water-soluble, then let it leave.
Two phases, one goal
Metabolism is traditionally split into two acts. Phase I (phase I metabolism) does *functionalisation*: it adds or unmasks a reactive handle — most often by sticking an oxygen onto the molecule. Phase II (phase II metabolism) does *conjugation*: it bolts a big, water-loving group onto that handle, like attaching a heavy, soluble anchor that the kidney can grab. The pattern is *first make a handle, then hang a soluble weight on it*.
Not every drug goes through both. Some are excreted unchanged. Some skip straight to phase II if they already carry a usable handle, like a hydroxyl. But the mental model holds: the body is patiently building water-solubility, one chemical step at a time. How fast it builds it is the drug's metabolic stability — a property you will fight to protect for the rest of this track.