Methylhistamines are not a recognized medical term or a specific medical condition. However, the term "methylhistamine" may refer to the metabolic breakdown product of the antihistamine drug, diphenhydramine, which is also known as N-methyldiphenhydramine or dimenhydrinate.

Diphenhydramine is a first-generation antihistamine that works by blocking the action of histamine, a chemical released during an allergic reaction. When diphenhydramine is metabolized in the body, it is converted into several breakdown products, including methylhistamines.

Methylhistamines are not known to have any specific pharmacological activity or clinical significance. However, they can be used as a marker for the presence of diphenhydramine or its metabolism in the body.