DIAGNOSIS: Mold, mildew, and dried toothpaste residue accumulating in the handle's socket and on the metal drive shaft. It's a major hygiene issue.
FIX: Remove the brush head. Use a stiff, dry toothbrush to scrub the metal shaft and the surrounding plastic. Then, dip a cotton swab in rubbing alcohol (isopropyl alcohol) and clean the entire socket area. Let it air dry completely before reattaching a head.
IF THAT DIDN'T WORK: For severe, caked-on residue, use a pin or needle to gently scrape the groove around the base of the metal shaft. Do not bend the shaft. Perform this cleaning monthly to prevent buildup.
HEADS UP: Never store the brush with a wet head attached. Always remove the head, rinse both parts, shake off excess water, and store separately to air dry. This prevents 90% of this problem.