Most runners either wear technical boxer briefs made from moisture-wicking fabric or skip underwear entirely by choosing shorts with a built-in liner. Cotton is the one thing almost every experienced runner avoids. Beyond that, the “right” choice depends on your anatomy, distance, and how prone you are to chafing.
Built-In Liners vs. Separate Underwear
Running shorts come in two camps: lined and unlined. Lined shorts have an interior layer sewn in, usually a brief-style or compression-style mesh, that replaces the need for underwear altogether. These liners are designed specifically for running, with moisture-wicking fabric, anti-odor treatments, and a snug fit that keeps everything in place. Compression-style liners (which extend down the thigh like boxer briefs) tend to outperform brief-style liners on longer runs because the edges of brief-cut liners can dig into skin and cause chafing in the inner thigh.
Runners who prefer unlined shorts typically pair them with high-quality technical boxer briefs. The argument here is flexibility: a good pair of boxer briefs is soft, form-fitting, and comfortable for hours, and you can wear them under any shorts you own rather than being locked into expensive running-specific models. There’s also a practical laundry benefit. After a short run, you can toss the boxer briefs in the hamper and get another wear out of the outer shorts.
The internal debate among serious runners isn’t really “underwear or no underwear.” It’s boxer briefs versus compression-lined shorts, since both offer similar coverage and support. Whichever route you pick, the goal is the same: a snug second layer that doesn’t shift, bunch, or hold moisture.
Why Fabric Matters More Than Style
Cotton absorbs and holds moisture like a sponge. In one study measuring sweat migration through clothing during a 50-minute run, a cotton garment reached 41% of its total absorption capacity. Synthetic fabrics absorbed substantially less moisture under the same conditions. That retained sweat creates a warm, wet layer against your skin that leads directly to chafing, irritation, and bacterial growth.
Technical running underwear uses synthetic blends (typically nylon or polyester with a small percentage of spandex) or merino wool. These fabrics pull sweat away from the skin and allow it to evaporate rather than pooling against your body. Merino wool has natural odor resistance and temperature regulation but costs more and dries slightly slower than synthetics. For most runners doing standard training distances, a synthetic blend is the practical default.
Some running underwear incorporates silver ions embedded permanently into the fabric. When activated by sweat, these charged silver molecules attract and neutralize the bacteria responsible for odor. This is genuinely useful for runners who train daily and don’t want to wash every piece of gear after every session, or for multi-day events where you’re packing light.
How Seam Construction Prevents Chafing
The biggest source of friction isn’t the fabric itself but the seams. Traditional stitching overlaps two layers of fabric, creating a raised ridge that rubs against skin thousands of times per mile. Flatlock stitching solves this by connecting fabric edges without overlap, producing a completely flat surface. High-friction zones like inseams and the groin area benefit the most from this construction. If you’re comparing two pairs of running underwear at the same price, the one with flatlock seams will almost always be the better choice for anything beyond a quick three-miler.
Some premium brands go further with laser-cut edges on leg openings and waistbands, eliminating seams entirely in those areas. This matters most for runners covering long distances where even minor irritation compounds over time.
Compression Underwear for Longer Runs
Compression-style underwear and shorts apply graduated pressure to the muscles of the upper leg. Research published in Medicine and Science in Sports and Exercise found that lower limb compression garments reduce muscle displacement and soft tissue vibrations during running across multiple speeds. Your muscles naturally oscillate with each foot strike, and compression dampens that bounce, which in turn reduces the amount of muscle activation your body needs to stabilize itself.
The practical takeaway: compression won’t make you faster (the same study found no improvement in running economy), but it can reduce that heavy, shaky feeling in your legs during and after long efforts. Runners training for half marathons or longer often gravitate toward compression liners or compression boxer briefs for this reason. The snug fit also eliminates ride-up, which is one of the most common complaints about looser underwear styles during runs over an hour.
Considerations for Women
For female runners, underwear choice has health implications beyond comfort. Non-breathable synthetic fabrics trapped against the vulvar area increase skin temperature, moisture, and pH, all of which can shift the local microbiome and raise the risk of bacterial vaginosis and yeast infections. One study of 500 women found that 60% of those diagnosed with bacterial vaginosis preferred tight underwear, while 72 to 82% of healthy women chose looser fits.
This creates a tension for runners: you want a snug fit to prevent chafing, but you also want breathability in the gusset area. The best compromise is underwear made from moisture-wicking synthetic or merino on the outer layer with a cotton or bamboo gusset panel, which provides vapor permeability and moisture absorption where it matters most. Many women’s running shorts now include liners designed with exactly this construction.
Thongs are a polarizing choice. Some female runners prefer them because there’s less fabric to bunch or shift, while others find the narrow back band causes its own friction issues. If you run in a thong, look for one made from technical fabric with flat or bonded seams rather than repurposing a cotton everyday thong. Bikini-cut or hipster briefs with laser-cut edges are the most popular middle ground for women who want minimal fabric without the thong tradeoff.
Considerations for Men
The primary concern for male runners is support and inner-thigh chafing. Loose boxers allow too much movement, leading to skin-on-skin friction and general discomfort within the first mile. Boxer briefs with a 5 to 7 inch inseam are the most common choice because they separate the thighs, keep everything supported, and stay in place with silicone-gripped or elastic-banded leg openings.
Pouch-style designs have become increasingly popular in running underwear. These use a separate anatomical compartment, often made of breathable mesh, that provides support without compression. The mesh keeps the area ventilated while the structured shape prevents skin-to-skin contact that causes chafing during repetitive motion. Combined with flatlock stitching at the inseam, this design addresses the two most common complaints male runners have about underwear.
What to Look For When Buying
- Fabric: Nylon-spandex blends or merino wool. Avoid cotton entirely.
- Seams: Flatlock stitching or bonded/laser-cut edges, especially along the inseam and leg openings.
- Fit: Snug enough to prevent shifting but not so tight it restricts circulation. You should be able to pinch about a half inch of fabric.
- Waistband: Wide, flat elastic that sits above your shorts’ waistband to avoid doubling up pressure on the same spot.
- Odor control: Silver-ion treatment or merino wool if you run frequently and want to extend wear between washes.
If you’re just starting out, the simplest approach is to buy a pair of lined running shorts and a pair of unlined shorts with technical boxer briefs. Run a few times in each setup and pay attention to where you feel friction, moisture, or bunching. Chafing patterns are highly individual, and finding your preference usually takes two or three trial runs rather than getting it perfect on the first try.