Complexes in powder form of iodine and a crosslinked polymer based on N-vinyl compounds are obtainable by dry heating of iodine and a polymer which is obtained by polymerization of monovinyl compounds (monomers A) whose vinyl group is bonded to a nitrogen atom of a nitrogen-containing heterocycle, in the presence of from 0.5 to 10% by weight, based on the monomers (A), of a compound (B) of the formula I; where A is —CH— or a nitrogen atom, and n is 2 or 3. The complexes are suitable for producing antidiarrheals.