Why Can't I Apply Latisse to Lower Lid or Eyebrows?

Latisse is bimatoprost, that is the same medication as the glaucoma drop, Lumigan that is formulated to lower the intraocular pressure. For many years, ophthalmologists have seen patients using Lumigan produce darker, thicker lashes. This is the basis of Allergan's repackaging of the same drug into Latisse.
So, Latisse will most certainly work to thicken and darken the eyebrow and and lower eyelid lashes.
However the caveat is that the skin will also become slightly red and have increased pigmenation in these areas as well. If you look at the photos on the Latisse website, you can see this increased redness and pigmentation. However, some may like this look, as it may simulate the look of eyeliner.
The next caveat to using Latisse off-label on the lower eyelid is that when Latisse gets onto the ocular surface (which occurs more easily with application to the lower eyelid), several things can happen. First, the eye pressure can be lower. This effect is harmless in virtually all patients. Secondly, and more importantly, the drug can actually enter into the eye and increase the pigmentation of the iris. A person with light brown eyes can darken to dark brow. At most risk would be patients with blue or green eyes, who might darken to a hazel color. Finally, patients who have ocular surface disease or are conteplating eye surgery such as cataract surgery or LASIK may actually increase their risk of complications if they have Latisse entering the eye.
The bottom line is that the off-label uses can be powerful, beautiful results, but check with your experienced provider first to clarify if you would be a good candidate or a more risky candidate for these other less desireable side effects.

Latisse is only recommended for use on the upper lash line. To use Latisse in another manner would be off label and I would not recommend doing that. 

The FDA has approved Latisse for use on the upper
eyelashes.At this time all other uses
are considered off label.Despite this
many practitioners have utilized Latisse on the lower eyelashes and eyebrows
with encouraging results.Under
these circumstances, many physicians are reluctant to use Latisse on the lower
eyelashes or eyebrow.With more study
the FDA will hopefully lift these restrictions.

It is recommended for the upper lash line only because there is some transference of the product to the lower lash line, every time you blink. This allows one bottle to last longer. It also reduces the risk of undesirable side effects when limiting usage, particularly dryness and itchiness.Latisse is only FDA approved for the eyelashes, however there are reports that it can achieve an improvement for brows as well.

There’s a longer explanation possible here, but the short version is this: the FDA has only approved LATISSE® to be marketed for use in the upper lashes. That being said, I am sure that some patients (and some doctors) have experimented with "off-label" uses of LATISSE® like thickening the eyebrows or lower lashes. Some of them have probably seen positive effects, too. That being said, though, it’s important to understand that LATISSE® has only been studied and approved by the results of clinical trials for use on the upper lashes, and there’s little information about the effects of using it elsewhere.

Wondering if Latisse can be applied on areas other than the upper lashes? The mechanism by which Latisse works suggest that it would work on any hair follicle on the body.  However, not all hair follicles on the body (eyebrows, eyelashes, beard, scalp, etc) have the same physiology leading to patterns of loss that differ and may make the formulation that works on upper lashes ineffective elsewhere.  Latisse was evaluated for and FDA approved for use on the upper lashes only.  Notwithstanding, I have personally seen patients independent of recommendations, apply Latisse to lower lashes and eyebrows and see some improvements.

You can apply latisse to your eyebrows, and they will probably become longer and thicker. 

You really don't need to apply latisse to your lower lids.  When you apply to your upper lids, some of the medicine does migrate to the lower lashes.  Also, if you apply to the lower lids, you are at more risk for irritation.

There are several potential complications of Latisse that are limited by restricting it's use to the upper lid.
Namely, darkening of the iris or the skin. It can also cause undesireable hair growth on the cheek.
 

Recommendations are to use Latisse as indicated in the initial FDA approval for hypotrichosis of the lower eyelids. Clearly, the FDA and Allergan will be closely evaluating an indication for the lower eyelids as well. At this time, lower eyelash use is off-label and in theory, should work as effectively as the upper eyelash. If you plan on using on the lower eyelashes, make sure you are receiving close follow-up by a physician.

Thanks for the question -
I would caution people a bit about using Latisse in off label applications. Specifically, the lower lashes get some of the medication when you close your eyes (the reason Allergan recommends using it at night). Placing medication on both lashes is likely to increase the dosage of the medication you're getting.
Dosage studies conducted by Allergan show that increasing the dosage or frequency of application does not change how quickly your lashes come in. What it does do is increase potential side effects (most commonly eye irritiation which can be serious).
Remember that Latisse is a drug, not a cosmetic. Becareful about using it in non-approved ways.
I hope this helps.