The right answer is whichever clean, well-fitting arrangement lets you run without rubbing, pressure, bunching, or unwanted movement. For some runners that is the liner already built into the shorts. For others it is separate sports underwear, compression shorts, or briefs designed for activity.
Check whether the shorts already have a liner
Many running shorts include an inner brief or short liner intended to replace underwear. Adding another layer can create extra seams, trap heat, or bunch. If the liner fits securely and feels comfortable, nothing else may be needed.
Choose support without compression problems
Different bodies and activities need different support. The garment should stay in place without cutting in, restricting normal movement, or creating numbness. Longer compression-style shorts can reduce thigh-on-thigh friction for some runners, while a light brief may feel cooler for others.
Put seams under suspicion
A small seam can become a large problem after thousands of repeated steps. Check the leg openings, waistband, labels, front and rear seams, and any point where damp fabric folds. Turn the garment inside out if necessary to understand what will touch the skin.
Use fabric that handles sweat
Light synthetic fabrics and wool blends generally dry more readily than heavy cotton. The goal is not a particular fibre; it is avoiding a saturated layer that stretches, moves, and rubs. Change out of wet clothing soon after the run.
Test the whole system
Shorts, liner, underwear, phone belt, tights, and hydration vest can overlap at the waist and create pressure. Test the complete setup on an ordinary run before using it for a long run or race. If chafing tends to occur, a compatible anti-chafe product may help, but it should not be used to disguise a persistently poor fit.
Replace what has stopped doing its job
Elastic loosens, fabric becomes rough, and seams distort. A favourite garment may eventually create problems it never caused when new. Retirement is appropriate when it no longer stays in place or when the fabric is damaged.
Running underwear is successful when you stop thinking about it.
Sharp pain, persistent skin damage, swelling, numbness, a new lump, urinary symptoms, or other concerning symptoms are not clothing-selection problems; seek appropriate medical advice.