It is important for the kallah to understand that a chuppat niddah can happen to anyone, even on medication. It is unfortunate but can be dealt with, and you can provide her with guidance so she has a better sense of it. The ultimate decision whether or not to take medication is a personal one.
In general, if a woman cycles regularly and other stresses have not changed her cycle in the past, then not intervening is a perfectly reasonable approach.
Still, stress — both positive and negative — can affect a woman’s hormones in the time leading up to a wedding. (Our sages recognized this effect as well.) We know of no formal studies of this phenomenon. Anecdotally, the percentage of women whose cycle is affected by their wedding is probably in the range of 5-20%.
Hormonal medications can cause staining. With combination pills, the risk of breakthrough bleeding during the first month of use is 15-70%. (The higher the estrogen level of the pill, the lower the likelihood of bleeding.) Such bleeding tends to abate over two to three cycles of use. There are no comparable statistics available for progesterone formulations like Primolut Nor, but the incidence of breakthrough bleeding is probably lower.
Primolut Nor is prescribed for kallot based on when they anticipate their period. If a woman learns fertility awareness at least 2-3 months before the wedding, she is more likely to be able to determine when her ovulation is over (based on basal body temperature). This may enable her to determine more accurately when she would expect to menstruate. That can help her decide whether to use Primolut Nor and, if she decides to do so, to optimize the scheduling.
If the kallah wants to consider taking medication, she should turn to a physician for a prescription, so that they can take her personal health history into account. Since Primolut Nor is used the month of the wedding, she has time to decide if she will use it. She can also revisit the issue should her cycle change in the near future.
Please write back with any further questions.