Across rivers and lakes worldwide, the antibiotic sulfamethoxazole has quietly accumulated, a byproduct of modern medicine that water systems were never designed to absorb. Researchers from three Chinese institutions have answered this slow-building crisis with a material that traps cobalt inside porous carbon beads, allowing it to destroy the antibiotic without releasing metal contamination in return. In doing so, they have navigated one of water treatment's oldest dilemmas: that the cure must not become a new form of the disease.