As for the usage of a pill (generic sense) or other medical procedure that has a contraceptive effect. The intention must not be contraceptive. If it has any chance of being an abortifacient then it must not be used or abstinence must be used. If it increases the risk of cancer greater than what is being prevented then it would be contraindicated for use.
It is kind of funny that the root of encyclical means circle (see Dismas2004 statement above).
What is below is the best thing out there explaining the authority of an encyclical and its infallibility specifically. There are four layers of infallibility. Immaculate Conception is a number one (ex cathedra proclamation). Against contraception is a two (historical, with founding, matter of faith and morals, that the magesterium is a competent authority to expound on). Humane Vitae fills these requirements. You can question (out of good conscience) to understand further on these two layers but submission and obedience to these two are necessary to be faithful. In other words, you can strive to understand but if you never understand you must still follow them.
An encyclical is pastoral to a specific concern. Not everything in it is automatically infallible but does require submission as a servant of Christ under the juridstiction of the magesterium (ordinary). However, with certain language (from above) infallible matter can be proclaimed/elevated/reaffirmed.
Hopefully this clears it up a little.
Under the Mercy,
Matthew
Hail Mary…
Quote from
newadvent.org/cathen/05413a.htm