I have been on CoCodamol for a long time and like a lot of things you do get used to them and they lose effect.
My GP prescribed Pentazocine. When I read the leaflet it went way beyond the "it may cause headaches or drowsiness". This is the cut down list of side effects
Cardiovascular: hypertension, tachycardia, bradycardia, hypotension, palpitations.
Central and peripheral nervous system: Hallucinations, dysphoria, disturbances of vision, disorientation, mood changes, nightmares, insomnia, grand mal convulsions , raised intracranial pressure, confusion, tremor.
And of all things - hypothermia!
It does make you wonder how this drug ever got through testing