Risk for gastrointestinal adverse events in the form of nausea, vomiting and abdominal pain motivates vigilance against insufficient intake of food, especially of carbohydrate.

Nausea, abdominal pain, diarrhoea are very common side effects of methylnaltrexone bromide that may be confused for an acute porphyric attack. These side effects may potentially be porphyrinogenic if leading to a decrease in carbohydrate intake.

Methylnaltrexone bromide is not metabolized by CYP enzymes, and has not shown the capacity to induce or inhibit any CYP enzymes.

Quartinary amine based on naltrexone (Cyclopropylmethyl epoxydihydromorphinan).

Methylnaltrexone bromide is indicated for the treatment of opioid-induced constipation when response to laxative therapy has not been sufficient in adult patients, aged 18 years and older. It is administered subcutaneously.

Methylnaltrexone bromide is mainly eliminated as the unchanged active substance, while a smaller portion is converted to less active methyl-6-naltrexol isomers by aldo-keto reductase 1C enzymes (Micromedex). According to the SPC methylnaltrexone bromide does not affect the pharmacokinetics of other drugs metabolized by cytochrom P450 enzymes. In vitro studies suggest that it does not inhibit the activity of CYP1A2, CYP2E1, CYP2B6, CYP2A6, CYP2C9, CYP2C19 or CYP3A4, but is a weak and clinically insignificant inhibitor of CYP 2D6 (SPC). One in vivo study showed that naltrexone does not appear to induce or inhibit metabolic activity of CYP2C9 or CYP3A4 (Adams 2005).