Is the use of contraception for medical purposes, by a married sexually active woman, justified by the principle of double effect?

No. The use of contraception is intrinsically evil and always gravely immoral. The principle of double effect never justifies an intrinsically evil act. Nothing can justify an intrinsically evil act because such an act is, in and of itself, morally illicit.

"Principle of Double Effect
"An action that is good in itself that has two effects--an intended and otherwise not reasonably attainable good effect, and an unintended yet foreseen evil effect--is licit, provided there is a due proportion between the intended good and the permitted evil….

"The object of the act must not be intrinsically contradictory to one's fundamental commitment to God and neighbor (including oneself), that is, it must be a good action judged by its moral object (in other words, the action must not be intrinsically evil)…."
Ascension Health, Healthcare Ethics, Key Ethical Principles

The principle of double effect may justify an act only if the act is 'good in itself,' in other words, only if the act is not intrinsically evil. Whether an act is 'good in itself' or 'evil in itself' is determined by its moral object. This 'object' is the end in terms of morality toward which the act itself is inherently directed (or intrinsically ordered). When an act is inherently directed toward a morally evil end, then that act is inherently evil. So the moral object determines the essential moral nature (or inherent moral meaning) of the act, in and of itself, apart from intention (or purpose) and circumstances (including consequences).

Pope John Paul II: "These are the acts which, in the Church's moral tradition, have been termed 'intrinsically evil' (intrinsece malum): they are such always and per se, in other words, on account of their very object, and quite apart from the ulterior intentions of the one acting and the circumstances. Consequently, without in the least denying the influence on morality exercised by circumstances and especially by intentions, the Church teaches that 'there exist acts which per se and in themselves, independently of circumstances, are always seriously wrong by reason of their object'." (Veritatis Splendor, n. 80; inner quote from Reconciliation and Penance, n. 17.)

Furthermore, the type of contraception which is used for medical purposes (e.g. to control an irregular period with excessive bleeding) is abortifacient contraception. This type of contraception can prevent conception by preventing ovulation. But it can also prevent the implantation of a conceived prenatal, causing the death of that very young human person.

Abortifacient contraception has two evil moral objects, to deprive the sexual act of the procreative meaning, and to deprive the innocent prenatal of life. Both moral objects are intrinsically evil and always gravely immoral. The evil of abortion is worse than the evil of contraception. Intrinsically evil acts are never justified by intention or circumstances. Therefore, under the second font of morality, the act is a grave sin.

In addition, the harm done by the abortifacient action of the pill, especially if the woman is on the pill for an extended period of time so that a number of innocent prenatals are killed, far outweighs the good done by the medical effects of that same pill (regulating the woman's cycle so as to provide some therapeutic benefit). In the third font of circumstances, the harm done by killing innocent prenatals gravely outweighs the medical benefits of the use. Therefore, under the third font of morality, the act is a grave sin.

Under the first font of intention, the intended end of obtaining the medical benefits of the contraceptive pill (as a therapeutic intervention) is good. However, a person's intention includes not only the intended end, but also the intended means. In this case, the woman intends a good end (health), but by a means that is intrinsically evil. The intention to use an intrinsically evil act as a means to a good end is an evil intention. Therefore, under the first font of morality, the intention to use an abortifacient contraception while remaining sexually active is an evil intention, despite the good intended end. For, even as concerns intention, a good end never justifies an evil means.

In summary, the principle of double effect only justifies an act if the act has a good intention, and the act is not intrinsically evil, and the bad consequences (effects) do not outweigh the good consequences (effects). An act that is justified by the principle of double effect is an act that has three good fonts. But when abortifacient contraception is used for a medical (therapeutic) purpose, all three fonts of morality are bad. Even one bad font is sufficient to cause any act to be a sin. In this case, all three fonts are gravely immoral. Therefore, it is in no way justifiable for a sexually active woman to use abortifacient contraception for a medical purpose.