P.r.n.

Abbreviation meaning 'when necessary' (from the Latin 'pro re nata', for an occasion that has arisen, as circumstances require, as needed).

One of a number of hallowed abbreviations of Latin terms that have traditionally been used in prescriptions.</P> Some others: > >a.c.

= before meals (from 'ante cibum', before meals) >b.i.d.

= twice a day (from 'bis in die', twice a day) >gtt.

= drops (from 'guttae', drops) >p.c.

= after meals (from 'post cibum', after meals) >p.o.

= by mouth, orally (from 'per os', by mouth) >q.d.

= once a day (from 'quaque die', once a day) >q.i.d.

= four times a day (from 'quater in die', 4 times a day) >q._h.: If a medicine is to be taken every so-many hours (from 'quaque', every and the 'h' indicating the number of hours) >q.h.

= every hour >q.2h.

= every 2 hours >q.3h.

= every 3 hours >q.4h.

= every 4 hours >t.i.d.

= three times a day (from 'ter in die', 3 times a day) >ut dict.

= as directed (from 'ut dictum', as directed) </LI></UL>