Some countries started to replace Pseudoephedrine by other drugs , such as phenylephrine . The reason for this is that pseudoephedrine can be used to make methamphetamine , which is an illegal drug . On November 23 , 2007 , the use and trade of Pseudoephedrine in Mexico was made illicit , as it was argued that pseudoephedrine was extremely popular as a precursor in the synthesis of methamphetamine .