I’m going to assume that if you have been trying to be faithful to the Church all these years, are in your 40s and have 5 kids that you must be doing something right and that you understand NFP and that it truly won’t work for you.
At your age, if your wife can not have children for medical reasons there are some who would say just abstain… that’s not reality nor what I believe the Church wants us to do. The renewing of the Marriage Covenant is very important to a happy marriage.
The Church does teach that oral contraceptives can be taken for medical reasons if the primary reason is not to contracept. Is that the medication your wife is on? If so then you are good.
If it is medication that is for other issues that will cause major birth defects I would submit a question on ask an apologist. Remember that even if NFP won’t work you should still be able to have relations right after her cycle is ended with very little chance of getting pregnant or during her cycle. I know that’s only a few days per month but it’s better than nothing.
Me personally I’m in a similar boat right now… but we are just almost 40 and have 3 children, 4th in a few week. I know what the Church teaches but I also know many Catholics and some Priests who consider a vasectomy a option. For me I believe we have been blessed with children beyond our highest hopes but my wife has SVT and another pregnancy could kill her. I have to believe on some level that God would rather my sons have a mother and us be together than risk another pregnancy. Yea yea… I know what all the nay sayers will say and yea I have read West’s books.
So truly I don’t know exactly what to tell you except that I’ll pray for you to make the right decision, will you pray that I can do the same?
Joe