In organic chemistry, a methiodide is a chemical derivative produced by the reaction of a compound with methyl iodide.  Methiodides are often formed through the methylation of tertiary amines:
: RN + CHI → (CH)RNI
Whereas the parent amines are hydrophobic and often oily, methiodides, being salts, are somewhat hydrophilic and exhibit high melting points.  Methiodides exhibit altered pharmacological properties as well.  
Examples include:
* Cocaine methiodide, a charged cocaine analog which cannot pass the blood brain barrier and enter the brain
* Bicuculline methiodide, a water-soluble form of bicuculline
Tertiary phosphines and phosphite esters also form methiodides.