complexation

Complexation is when a drug temporarily teams up with a partner molecule, like two dancers loosely holding hands — joined enough to behave as a pair, but able to let go again. The pairing is reversible, held together by weak forces rather than permanent chemical bonds.

The drug and a complexing agent associate through forces such as hydrogen bonds, van der Waals attractions, hydrophobic effects or, in some metal complexes, coordination bonds. Because the resulting complex has different properties from the free drug, complexation can be used to increase apparent solubility, mask bad taste, improve chemical stability, slow release, or reduce irritation. The equilibrium between free and complexed drug is described by a stability (binding) constant.

A classic example is inclusion complexation with cyclodextrins, where the drug sits inside a ring-shaped sugar. Complexation is reversible, so on dilution in the body the complex dissociates and frees the drug for absorption — but a very strong complex can hold the drug too tightly and actually reduce its availability, so binding strength must be balanced.