Narrow-spectrum drenches generally just have activity against one or two species of sheep worms. It is preferable to use these against specific worms (e.g. an infestation of Barbers Pole Worm) rather than a broad-spectrum drench.
Long-acting treatments (injections or boluses) are useful treatment options when stock are in contaminated pastures. However, they can lead to rapid onset of resistance if misused.
A combination drench contains two or more active ingredients that each targets the same worms. This gives an increased chance that worms resistant to one active are killed by the other active. This can be useful as many farms have worms resistant to more than one active (Wormboss).