Constituents of a commercial teething preparation (Dr. Hand's teething lotion) are absorbed and can interfere markedly with measurement of serum diphenylhydantoin levels by the method of Svensmark and Kristensen.
Jones MD, Helfer RE. A Teething Lotion Resulting in the Misdiagnosis of Diphenylhydantoin Administration. Am J Dis Child. 1971;122(3):259–260. doi:10.1001/archpedi.1971.02110030117021