British National Formulary defines Ultralente-type insulins as: A sterile neutral (neutral used here refers to the pH, not to the fast-acting insulin known as neutral or R) suspension of bovine insulin or of human insulin in the form of a complex obtained by the addition of a suitable zinc salt; consists of rhombohedral crystals (10-40 microns).
Ultralente insulin crystals are large and slowly absorbed because of their size, making this a long-acting insulin.
Trade names:
- Humulin U or Humulin Zn No longer produced