Several commercial over-the-counter shampoo products contain active ingredients designed to target and alleviate dandruff. The main active ingredients used include salicylic acid, zinc pyrithione, coal tar, resorcine, selenium and ketoconazole, according to the National Institutes of Health. Though they are effective at combating dandruff, they may hold several potentially negative side effects.
Zinc Pyrithione Irritation
Some people may be allergic to zinc pyrithione, or may experience skin irritation. Both side effects are very rare, according to Drug Information Online. It may also irritate your eyes if it gets into them, and can cause nausea if it's swallowed, reports the National Library of Medicine's Households Products Database.
Coal Tar as Carcinogen
Coal tar works as both an anti-dandruff agent and a biocide. The Environmental Working Group's cosmetic safety database ranks it a 10 on a scale of 0 to 10, with 10 being the most hazardous to your health. It is a known carcinogen and may be toxic to your skin's health. It's banned in various geographic regions like Canada and the European Union, though it's available in over-the-counter shampoos in the United States.
Salicylic Acid Effects
Salicylic acid may help with dandruff by loosening dead skin crusts. You may feel a stinging or burning sensation when applying a shampoo containing the acid, and it may irritate your skin, according to the National Institutes of Health. More serious, potential side effects may include vomiting, dizziness and changes in breathing rates, though such effects are rare.
The U.S. Department of Labor's Occupational Safety & Health Administration says resorcinol may irritate your mucous membranes, such as the areas around your nose, as well as your skin and eyes. If you have wounds and get the chemical in them, it may also cause a rare blood disease known as methemoglobinemia.
Ketoconazole may prompt itching of your skin, hives and a rash when using topically, according to the National Institutes of Health. More serious side effects, such as an increased risk of cancer, are related only to internal ingestion of the chemical.
Some people may be allergic to selenium, warns the National Institutes of Health. The institutes also reports that skin irritation, rashes and other topical side effects are most often associated with shampoos containing selenium at a strength of 2.5 percent or higher; you may wish to avoid dandruff products that contain the chemical at such a strength.