The need to keep down levels of volatile isocyanate emitted from polyurethane adhesives often makes them highly viscous. There are ways around this, however, that as well as lowering viscosity can also improve adhesion to low surface energy substrates and reduce leaching of primary amine contaminants. When producing adhesives for use in flexible packaging, or reactive polyurethane hot-melts, low viscosity polyurethane prepolymers can be especially useful. When used as a basis to formulate from, they make it easier to create an adhesive that can be readily dispensed, even at lower temperatures. Some polyurethane prepolymers can be used as adhesives themselves on surfaces that contain moisture, and therefore lowered viscosities would allow especially easy dispensing. Alternatively, two component systems can be created, using a polyol, polyamine, polyacid or similarly reactive chemical as the second, hardening, component.