I am also slightly worried about the pills causing changes to her libido and health in general but she told me that the pill has been tested rigorously and is perfectly safe.
There are different kinds of birth control pills. Most contain estrogen and progesterone. But the amounts of each hormone probably vary a bit. Some can completely prevent periods, others allow a period, but no ovulation, of course.
I found out in my early 40s, when I'd gone back on the bc pills, that I had a blood mutation, which caused me to be 7 times more likely to get a blood clot in my legs, or lungs, or indeed, in my brain, which would cause a stroke. I found this out because I was hit in the ankle hard in a shopping cart accident, and it caused me to get clots in one leg and both lungs. I found out that hormonal birth control pills containing estrogen also cause a woman to be 7 times more likely to clot than otherwise. So I was a ticking time bomb, 49 times more likely to clot than another person.
A side effect of bc pills containing estrogen are to cause your blood to coagulate much more easily. For most women, this wouldn't be a huge risk. I was on bc pills from age 16-29 and never had a problem. But the risk gets higher as you age. (I began using a diaphragm at age 29 when we started having kids, that is, in between my three pregnancies. During lactation I felt a diaphragm was safe enough, as we did nursing on cue, into toddlerhood or preschool age, and I went without periods for years on end.)
My doctor advised me to have my teen daughters tested for this blood mutation after I got the blood clots. One was positive for it, the other was not. The one that has the blood mutation is only supposed to use the "mini-pill" with progestin only (if she chooses to use bc pills). A drawback of those pills is you must take them at the exact same time every day.
I also found that being on the pill did reduce my libido quite a bit, when I was on it as a young woman.
I disagree that bc pills are "perfectly safe."
All drugs have side effects.
Nowadays, the IUD has become quite an attractive alternative, with the advancements in design.