molecular weight

Molecular weight is simply how heavy one molecule is, found by adding up the masses of all its atoms, expressed in daltons or grams per mole. It is the most basic size yardstick in drug design: roughly, the bigger the number, the bigger and bulkier the molecule.

Size matters because larger molecules tend to permeate membranes more slowly, dissolve less readily, and are harder to absorb orally. Small-molecule drugs typically fall below about 500 daltons, the threshold enshrined in Lipinski's rule of five, though many successful oral drugs sit somewhat above it.

Molecular weight tends to creep upward during optimization, because adding atoms is an easy way to boost potency. Watching it (and the related count of heavy atoms) keeps a project from drifting into 'molecular obesity', where extra bulk buys potency at the expense of solubility, permeability, and overall druglikeness.