Prescription guidelines should be changed after a man died from using two medicines together, a coroner warned.
Graham Saffery, 48, from Bedford, was admitted to hospital after being prescribed an antidepressant and a painkiller that could interact badly.
After two days of intensive care, the drugs were restarted and he was found dead at home three weeks later.
A coroner's report said advice "relied upon" by doctors failed to mention the "known risk" of combining these drugs.
An inquest heard Mr Saffery was prescribed the painkiller oxycodone in October 2016 after a road traffic accident and given amitriptyline for depression in January 2018.
A pharmacist reported he appeared "drugged and confused" in April 2018 but his antidepressant dosage was increased, leading to hospital treatment.
Emma Whitting, senior coroner for Bedfordshire and Luton, confirmed Mr Saffery died as a result of taking a combination of the drugs, which was "known to carry a risk of over-sedation".
'We have listened'
In a report sent to the National Institute for Health and Care Excellence (Nice), which issues guidance to the medical profession, she warned of the risk of future deaths unless action was taken.
She said: "Although other pharmacological guidance recommends the need for both caution and monitoring when prescribing amitriptyline and oxycodone simultaneously, such advice does not appear to be provided by the British National Formulary (BNF), which is regularly consulted and relied upon by GPs."
Nice said it was not responsible for content of the BNF - a pharmaceutical reference book - and referred the BBC to the Royal Pharmaceutical Society (RPS).
An RPS spokeswoman said: "We are committed to providing the most useful information to healthcare professionals and, as such, the content of all our publications, including the BNF, is always subject to review."
East London Foundation Trust said it provided mental health support to Mr Saffery and he was visited at home by crisis services.
A spokesman confirmed it carried out a "detailed investigation" into his death.
"We have listened, learnt and made changes where necessary in light of this investigation," he added.
Bedford Hospital said its staff had followed "all necessary processes".