Common adverse effects include irritation of the skin at the application site, such as dryness, stinging, itching and peeling.

[4] Generalised allergic reactions are rare; they include hives as well as swelling of lips, tongue, and throat, potentially closing up the respiratory tract.

[4] Ingesting more than a few grams can lead to poisoning by hydrogen sulfide (H2S), which the gut flora produces from sulfur.

H2S is also generated on the skin, but topical formulations do not induce toxic concentrations, except when large quantities are used for small children.

[1] Sulfur's keratolytic action is also mediated by H2S; in this case, the hydrogen sulfide is produced by direct interaction with the target keratinocytes themselves.