Can we use contraception for medical reasons?

Your priest is badly misinformed. The deliberate use of contraception by spouses to prevent pregnancy is never licit (CCC 2399).

Humanae Vitae (14) rejects the arguments that a lesser evil may be chosen, or that contraceptive acts merge with past and future fertile acts into a single morally good whole. It is sometimes lawful to tolerate a lesser moral evil to avoid a greater one or promote a greater good, but never lawful, even for the gravest reasons, to do evil that good may come of it: to intend directly something that by its nature contradicts the moral order, even to protect the welfare of an individual, family or society. So it is a serious error to think that a married life of otherwise normal relations can justify deliberately contraceptive intercourse, which is intrinsically wrong.