Abstract: A thumb ring insert that reduces the effective size of a syringe thumb ring thumb passage. The insert comprises a thumb aperture adapted to receive a user's thumb in a comfortable fit with the aperture eccentric from the thumb ring passage, located close to a thumb ring bottom where an end of a syringe plunger connects to the thumb ring. Thus, range of movement between aspiration and expression of the syringe is reduced as free float of the thumb in the thumb ring is effectively eliminated. With the insert aperture close to the thumb ring bottom, the expanse between the user's thumb and fingers is also reduced when aspirating. The insert further comprises a moderately pliable circumference groove about the insert that matches the syringe thumb ring receivable therein by snapping the insert into place in the thumb ring passage with groove rims being resiliently distorted in allowing the thumb ring to pass into the groove.