From Jone --

c) Since morphine, opium, chloroform and similar
drugs can also deprive one of the use of his reason temporarily,
that which was said of intoxicating drinks holds
also for narcotics (Cf. 165, 4).
a) To use narcotics i n small quantities and only
occasionally, is a venial sin if done without a sufficient
reason. Any proportionately good reason justifies their
use, e.g., to calm the nerves, dispel insomnia, etc.
Such usc becomes gravely sinful if it creates an habitual craving
for "dope" which is more difficult to overcome than dipsomania
and more injurious to health.
(3) To use drugs in greater quantities so as to lose
the use of one's reason is in itself a mortal sm; but for
a good reason it is permissible.
Such a good reason is had in case of operations, i.e., that the
patient be rendered insensible to intense pain, or that one might
remain calm under the knife. In like manner one may administer
opiates to one who is suffering greatly in order to alleviate his pain.
y) In general it is forbidden to make a patient unconscious
by the use of drugs in non-lethal doses in order
that he may have a painless death (therapeutic euthanasia
2 1 1 ) .
Such action i s lawful i f the sick person i s well prepared for
death and there is danger that he might otherwise fall into sin.
Some authors allow such a procedure if the dying person is thus
spared unusually great suffering and if one has reason to presume
his consent. - According to the general opinion such a practice
is not allowed merely to remove the ordinary anxieties that accompany
the death agony. If the patient asks for such drugs in good
faith and if there is no hope of teaching him otherwise, he should
be left in good faith.
Such a practice is never allowed if the sick person is not
prepared for death, and hope remains that he might eventually
prepare for it. In such a case one must oppose as far as he can
the doctors and relatives who desire to effect his dying while
unconscious.