Actually, the stance according to Christopher West’s book “Good News about sex and Marriage” which explains the writings of the Pope regarding chastity and birth control says that:
“First, let me state the moral principal involved, then I’ll address the pill specifically. as Pope Paul VI said in Humanae Vitae: ‘The Church does not at all consider illicit the use of therapeutic means truly necessary to cure diseases of the organism, even in an impediment to procreation, which may not be foreseen, should result therefrom, provided such impediment is not, for whatever motive, directly willed. ‘
Suppose a woman had a hysterectomy to remove cancer, subsequent sexual acts would be obviously sterile, but the intention of the operation was to remove her cancer, not to sterilize her (A Medical reason to avoid pregnancy does not justify sterilization) The fact that she is now sterile is an unfortunate but unintended consequence of the procedure.
Taking the PILL for medical reasons would be similarly acceptable, but for the fact that the pill is a potential abortifacient. If there were absolutley no medical alternatives for a mrried woman on the pill for therapeutic reasons, she and her husband would have to abstain from sex COMPLETELY in order to avoid the risk of abortion. The good news is, there are many medical alternatives to the pill.
[West; Good News about Sex and Marriage, p.128]
I would HIGHLY encourage you to read the above mentioned book. It cleared up a lot of my confusion regarding catholic practices.