What logP really measures
Lipophilicity is, plainly, how much a molecule "likes oil over water." We measure it with a classic experiment: shake the compound in a sealed vial holding equal volumes of water and octanol (an oily solvent), let it settle into two layers, and ask where the molecule went. logP is the base-10 logarithm of its concentration ratio between octanol and water. A logP of 2 means the molecule sits 100-fold more in oil than in water; a logP of 0 means it splits evenly; a negative logP means it prefers water.
logP = log10( [compound in octanol] / [compound in water] ) Example readings: caffeine logP ≈ -0.1 (slightly water-loving) aspirin logP ≈ 1.2 (balanced) ibuprofen logP ≈ 3.5 (fairly greasy) a typical oral drug logP ≈ 1 to 3 (the comfortable middle)
logD: lipophilicity at a real pH
logP describes a single, neutral form of the molecule. But many drugs contain acidic or basic groups that gain or lose a charge depending on the surrounding pH — a topic we cover fully in the next guide on pKa and ionization. A charged molecule is far more water-loving than its neutral twin. So at body pH, the *effective* lipophilicity can be very different from logP.
logD fixes this. It is the same oil-vs-water partition, but measured at a specified pH (usually 7.4, the pH of blood), accounting for all forms of the molecule — charged and neutral together. For a compound with no ionizable groups, logD equals logP. For an acid or base, logD at 7.4 is lower than logP because the charged fraction hides in the water layer. When someone quotes a single "lipophilicity" number for an oral drug, they almost always mean logD7.4 — it is the more honest, body-relevant value.
The narrow sweet spot
Lipophilicity pulls in two directions. Too low (very polar) and the molecule won't cross membranes — poor permeability and weak absorption. Too high (very greasy) and several bad things happen at once: solubility collapses, the molecule sticks to off-targets and to metabolic enzymes, and the risk of toxicity climbs. Decades of data show most oral drugs cluster in a comfortable band of logP roughly 1–3, or logD7.4 around 1–3.
Because adding greasy groups is the easiest way to gain potency — oil-loving surfaces bind tightly to oil-loving pockets — teams drift toward higher lipophilicity almost by accident. This creeping bloat is sometimes called molecular obesity. The cure is to measure potency *per unit of greasiness* using lipophilic ligand efficiency (LLE = pIC50 − logD), which rewards potency that comes from genuine, specific interactions rather than from sheer oiliness.