What it means
A handwritten U can be mistaken for a 0, a 4 or "cc," turning 4U into 40 or 44, a tenfold or greater overdose; this matters most with insulin and heparin, which are dosed in units. The Joint Commission's "Do Not Use" list and ISMP both ban U (and u). The same goes for IU, international unit.
For the nurse
If an order says U, clarify it with the prescriber before giving the drug. Always write "units" yourself.
Related terms
insulin
A hormone made by the pancreas that lets the body's cells take in glucose, also made as a medicine.
heparin
An anticoagulant drug, given by injection or IV, that keeps blood clots from forming or growing.
leading zero
A zero written before the decimal point of a number less than one, as in 0.5 mg.
trailing zero
A zero written after the decimal point in a whole-number dose, as in 1.0 mg; it must not be used.