Polymyxin B & Colistin

How it works

Last-resort antibiotics whose charged ring and fatty tail pierce the outer membrane of gram-negative bacteria. Hard on kidneys.

What it's made of

Two nearly twin antibiotics made by a soil bacterium: a positively charged ring of amino acids and a fatty tail. Colistin is injected as colistimethate, an inactive form that the body converts.

What it's for

Prescribed for

Serious infections caused by gram-negative bacteria with few options. In Spain, inhaled colistimethate is also prescribed for chronic Pseudomonas infection in cystic fibrosis.