Let’s start by defining our terminology. Contraception is the intentional act of rendering marital relations temporarily or permanently sterile. When contraception is not willed, a person is not engaging in it. In this case, the inquirer is asking whether she can use a particular drug – popularly called “the pill” – for the specific purpose of treating endometriosis. The fact that the particular drug also renders marital relations sterile is, in this situation, an unintended side effect. It would be similar to taking another drug for a heart condition that had the unfortunate side effect of rendering marital relations sterile. Because the side effect is not intended, only accepted as a by-product of treating the heart condition, the person is not engaging in contraception if she continues to have marital relations with her husband.
Here is a definition of abortion by Pope John Paul II in Evangelium Vitae:
Therefore, by the authority which Christ conferred upon Peter and his successors, in communion with the bishops – who on various occasions have condemned abortion and who in the aforementioned consultation, albeit dispersed throughout the world, have shown unanimous agreement concerning this doctrine – I declare that direct abortion, that is, abortion willed as an end or as a means, always constitutes a grave moral disorder, since it is the deliberate killing of an innocent human being. This doctrine is based upon the natural law and upon the written word of God, is transmitted by the Church’s Tradition and taught by the ordinary and universal magisterium (EV 62; emphasis added).
Thus, abortion must be willed as an end (i.e., one intends to kill a child), or used as a means to obtain some other end (i.e., one kills a child in an abortion to further some other goal), to be abortion. If one actually performed an abortion in order to treat endometriosis, such an action would be a moral evil falling under the proscriptions of Evangelium Vitae and the two-thousand-year teaching tradition of the Church. But is that the case in this particular situation?
In this case, using this particular drug (“the pill”) as a treatment for endometriosis falls under the realm of moral law known as double-effect. Among the various side effects of the drug is that it renders marital relations temporarily sterile and it has a potential to be abortifacient. Neither of these side effects are intended, only accepted as by-products of treating the disease in question. If the need to treat the illness is proportionate to the treatment required, one can use even a treatment that is known to have the side effects outlined. It may be that there are other treatment options available, which is why I recommended the work of the Pope Paul VI Institute for the Study of Human Reproduction for medical bioethics questions in the realm of women’s health issues, but that does not mean that a person using this particular treatment option is committing abortion thereby.