Derfen has instant effects against SSRD ; results will be noticeable on any parts of the horse.

If you notice a difference of results between areas, there may be several reasons for it:

1-Quantity of Derfen applied is not enough
2-Intervals between 2 applications are too long
3-your horse may be reacting to something else than SSRD. Indeed, worming may not be up-to-date (thus leading to tail itching) or your horse may sweat excessively and so its hair gets sticky; the horse will then rub on these specific areas(in such situation, there is nothing to worry about ). Other reasons for rubbing specific areas such as forelock is that your horse is getting bored, or it is allergic to certain types of grass.
Last but not least, it may bite its flanks because of internal problems (kidney, digestion) or insects' bites.

If you follow instructions to apply Derfen (quantity, intervals between applications, shaking the bottle prior to another application), itching will stop immediately no matter how affected your horse is and whatever type of horse or environment it may be.