Pope Benedict XVI on Saturday acknowledged that the Vatican's instructions against using birth control is complicated, even as he praised a 1968 Church document that condemned contraception.

In a speech marking the 40th anniversary of the document, Benedict reiterated the Church's ban against artificial birth control as well as more recent teaching against using artificial procreation methods.

Pope Paul VI's 1968 "Humanae vitae" ("On Human Life") encyclical prohibits Catholics from using artificial birth control.

"The teaching laid out in the 'Humanae vitae' encyclical isn't easy," Benedict said.

"What was true yesterday remains true even today. 'The truth expressed in 'Humane vitae' doesn't change; on the contrary, in the light of new scientific discoveries it is ever more up to date," the pope added.

Benedict appeared to be referring to artificial procreation methods, which in the Church's view offend the dignity of life and go against Vatican teaching that the only way to conceive a child is through intercourse between husband and wife.

"No mechanical technique can substitute the act of love that two married people exchange as a sign of a greater mystery," Benedict said in his speech.

Benedict expressed concern that human life risks losing its value in today's culture and worried that sex could "transform itself into a drug" that one partner had to have even against the will of the other.

"What must be defended is not only the true concept of life but above all, the dignity of the very person," the pope added.

Paul VI was said to have agonized over whether to allow artificial conception in preparing the encyclical. Benedict described Paul's decision as the fruit of much suffering and the document as "a significant gesture of courage."

"Forty years after its publication, that teaching not only shows itself to be unchanged in its truth, but it reveals the farsightedness with which the problem was tackled," Benedict said.