Hormone Replacement Therapy, or HRT, is used primarily in women when their natural hormone levels are significantly changed. The two biggest groups of women receiving HRT are perimenopausal (those going through menopause). They are having trouble adjusting to or living their life with their new hormonal levels—those women who have had a hysterectomy (especially those whose hysterectomies happened at a younger age).
When the female reproductive organs go through menopause or are removed, they significantly impact the regular production of hormones, particularly estrogen and progesterone. One of the first signs of reduced estrogen levels in the body can be a decrease in vaginal lubrication, leading to vaginal dryness.
This vaginal dryness is one of the reasons many women choose to go on HRT. However, it also relieves symptoms like hot flashes, moodiness, the slowing onset of or reducing the chance of having osteoporosis, etc.
Postmenopause, many of these symptoms disappear, although quite a lot of women continue to report experiencing vaginal dryness throughout their older ages. One easy way to deal with vaginal dryness, either with or without the use of HRT, is adding a little bit of a good quality lubricant into your sexual play.
Whether you chose a silicone-based lube or a water-based lube, adding lube with help to re-lubricate the vagina (and anus, if you choose to play there as well). A slick vagina or anus increases pleasure, reduces the friction that occurs during penetration (of fingers, toys, penises, etc.), and, very importantly, reduces the chance of vaginal or anal tissue tearing during sexual activity.
Not only can these micro fissions wind up being painful, but they can also leave the body more susceptible to the transmission of bacterial infections and even sexually transmitted infections (STIs).
Moreover, since HRT is not recommended for use for a long period of time, and is contraindicated in those who have migraines, survivors of breast or ovarian cancer, those with liver disease, CAD, and more, it's always great to have a bottle of lube on hand to keep things nice and slick whenever you want, regardless of whether you have HRT going on as part of your medical regime.
You can absolutely use lubricant when you are on HRT and before you are on HRT, and after you have completed your HRT treatment. Just make sure you are using a water-based lubricant or silicone-based lubricant, and you'll be ready to rock whenever you are ready!