Two molecules can have the same chemical formula and behave completely differently. Ethanol and dimethyl ether are both C2H6O. Ethanol is a liquid that mixes with water and gets you drunk. Dimethyl ether is a gas used as a propellant. Same atoms, different arrangement. They are isomers. The formula tells you what is there. The structure tells you what it does.

Isomers come in several types. Structural isomers differ in how the atoms are connected. Butane and isobutane are structural isomers. Stereoisomers have the same connectivity but differ in spatial arrangement. Cis-trans isomers differ in the geometry around a double bond or ring. Enantiomers are mirror images that cannot be superimposed, like left and right hands. They can have dramatically different biological effects. Thalidomide is the classic cautionary tale: one enantiomer treated morning sickness, the other caused birth defects.

Isomerism matters in drug design. A drug must have the right shape to fit its target. The wrong isomer may be inactive or harmful. Regulatory agencies now require drug makers to characterize and test each isomer separately.