Selenium sulfide will help knock out the excess scalp fungus (yes, gross), which can cause dandruff. This both soothes irritation and slows the rate of skin cell shedding. You'll see it in 1 percent or 2.5 percent concentrations in anti-dandruff shampoos. Look for it in Head & Shoulders Clinical Strength Shampoo, $7.99, which promises to remove 100 percent of flakes and relieve itchiness. But one warning: This ingredient has been known to discolor blonde, gray and dyed hair, so only use as instructed, and be sure to rinse well after shampooing.

Um, tar in our hair? Normally, we'd take a hard pass, but hear us out. Coal tar, a byproduct of the coal manufacturing process, doesn't sound pretty but it can attack dandruff caused by seborrheic dermatitis, eczema and psoriasis. This ingredient slows down the rate your skin cells turnover, which makes your scalp less likely to flake. Seek out products that contain 5 percent coal tar, such as PsoriaTrax Anti-Dandruff Tar Shampoo, $18.95.

When you're faced with stubborn dandruff that just refuses to go away with other dandruff shampoos, it's time to call on ketoconazole, the king of scalp antifungal agents. Ketoconazole treats a number of different types of fungal and yeast infections and is usually the ingredient used in prescription dandruff shampoos. You can find the active ingredient in OTC anti-dandruff shampoos, like Nizoral A-D Shampoo, $16.99.

Tea tree oil, from the Melaleuca alternifolia plant, is an ancient remedy for acne, athlete's foot and lots of other infections. Plus, research suggests that its fungus-fighting action may treat dandruff. If you don't find tea tree oil too strong, you can rub it directly into your scalp, let the oil sit for a few minutes and rinse well. Or, look for a dandruff shampoo that contains tea tree oil, such as Aubrey Organics Tea Tree & Primrose Shampoo, $14.98. This shampoo not only includes tea tree oil to fight fungus, it contains primrose oil to moisturize your dry scalp.