Pill inserts are small eccentric plastic or aluminum bushings pressed into RC suspension blocks (A, B, C and D mounts) to change hinge-pin angle and height in fixed steps.
Modern 1/10 and 1/8 off-road and dirt oval kits from Team Associated, TLR, XRAY and Tekno hold inner suspension hinge pins inside aluminum mounts fitted with replaceable eccentric pills, usually marked with dots or degree numbers such as 0.5 and 1.0. Rotating or swapping pills in the C and D rear mounts alters rear toe-in, anti-squat and inner hinge-pin roll center height without buying new blocks. Moving pills up raises the inner pivot and frees the car up in high-grip carpet; moving rear C-block pills out adds rear toe-in for forward traction on slick clay. Always write down both the dot size and arrow orientation on your setup sheet because one flipped pill changes handling across the whole corner.
One flipped or misread suspension pill changes rear toe, anti-squat or roll center by a full degree, making pill diagrams one of the highest-value fields on an RC setup sheet.