Can a married woman use the contraceptive pill for a medical purpose, and also use natural family planning (NFP) so that she may continue to have sexual relations with her husband?

No. Natural family planning is moral because it does not deprive sexual acts of the procreative meaning. NFP allows natural marital relations to retain the unitive, procreative, and marital meanings in the moral object. However, the use of the contraceptive pill does deprive sexual acts of the procreative meaning. And the use of NFP, while also using a contraceptive, does not remedy that deprivation. In other words, if you try to combine a contraceptive with natural family planning, the sexual acts remain deprived of the procreative meaning by the contraceptive, and so the use of the contraception remains intrinsically evil.

NFP is based on refraining from marital relations during certain times (e.g. times of increased fertility if the couple wishes to avoid conception), and engaging in natural marital relations open to life during other times. The use of artificial contraception causes any acts of marital relations to be closed to life, thereby making the attempt to use natural family planning not truly natural. NFP is effectively nullified whenever the couple uses any method of contraception along with a method of natural family planning.