As I often point out, in a physical and physiological sense, sex is good for us males (although we do have to be careful with certain often-deadly STD's). Of course, sex is good for females, too, and so that leads to some interesting questions: "Why are there so many drugs that can help males with sexual function?" and "Will those same drugs that help middle-aged and beyond guys possibly help middle-aged and beyond females as well?"
Of course, the drug companies have looked into this, since that would open up a whole new market for them and found that there is some potential there. This is not surprising since, as we have covered elsewhere, women have some analagous chemical, structural and circulatory similarities to us in their sexual response. The clitoris and vaginal walls, for example, are dependent on blood flow and nitric oxide during sexual activity to a significant extent. In my article on the G-Spot, I even cover have researchers have discovered erectile tissues in women.
Here's another important key: lubrication in women is tied to nitric oxide and blood flow as well.  In fact, studies have shown that women with heart disease, and therefore impaired endothelial function, have decreased vaginal blood flow and lubrication, leading sometimes to "sexual arousal disorder".  Not that disimilar from us guys, eh?
So, then, why not Viagra for women? Why can't women achieve the same boost in sexual performance that guys experience?
The answer is that Viagra (sildenafil) has had significant success already in the research. For example, one study of post-menopausal women found solid improvements in orgasms and clitoral stimulation from taking sildenafil and there was also an increase in mean - as in average - lubrication as well.  In fact, it is interesting to note that about one in eight women actually withdrew from this study because of "clitoral hypersensitivity". NOTE: It should be noted that one study of women with Female Sexual Arousal Disorder (FSAD) showed no improvement in sexual outcomes, however.  This may be why Viagra, Cialis and Levitra are not generally prescribed for female sexual dysfunction.
There is also evidence that another large class of women, those on traditional antidepressants, may be helped by sildenafil. Scientific American reported that about 11% of women (in the U.S.) were on an antidepressant , and, undoutedly, this is higher in the block of women (over 35+) that would likely consider or need treatment. In this huge category of women, literally tens of millions worldwide, Viagra has been shown to improve overall sexual function significantly. 
NOTE: Have her talk to her doc. Loss of libido and sexual function in women can be the results of various serious medical conditions.
Of course, you can probably guess what I am going to say at this point: instead of taking a drug, women can very likely experience similar improvement from the non-drug improvement of blood flow and nitric oxide that I recommend for guys. See this link on How to Improve Erectile Strength to know the kind of things I am talking about. Any of these items will likely improve clitoral sensitivity and sexual function in a much more natural way that avoids or limits the use of drugs.
In other words, if you've made significant lifestyle changes to improve your erectile strength, including exercise, diet and supplements, be sure to include the woman in your life. The changes that you are making will likely help her as well. Furthermore, not only will it help you grow closer, it will help her become healthier, the rewards of which will likely manifest ifself in the bedroom as well.
CAUTION: Women on Viagra commonly experience many of the same side effects as men, including stomach upset and headache. As far as I know, the lose-your-hearing and lose-your-sight side effects have not been reported.
1) International Journal of Impotence Research, 2003, 15:461 464, "Topical administration of a novel nitric oxide donor, linear polyethylenimine-nitric oxide/nucleophile adduct (DS1), selectively increases vaginal blood flow in anesthetized rats"
2) Eur Urol, 2000, 38: 20 29, "Anatomy and physiology of female sexual function and dysfunction: classification, evaluation and treatment options"
3) Urology, 1999 Mar, 53(3):481-6, "Safety and efficacy of sildenafil in postmenopausal women with sexual dysfunction"
4) Journal of Women's Health & Gender-Based Medicine, May 2002, 11(4):367-377, "Efficacy and Safety of Sildenafil Citrate in Women with Sexual Dysfunction Associated with Female Sexual Arousal Disorder"
5) Scientific American, Feb 2008, "The Medicated Americans: Antidepressant Prescriptions on the Rise"
6) JAMA, 2008, 300(4):395-404, "Sildenafil Treatment of Women With Antidepressant-Associated Sexual Dysfunction"