Dr Holdcroft with Tom Kelleher
Up to 2,000 patients at the Chelsea and Westminster Hospital in London will be taking part.
Some will get tablets containing the "cannabinoid" chemical, others conventional pain relief capsules and some others a placebo pill with no active ingredients.
The trial, led by Dr Anita Holdcroft, is another in a clutch of experiments launched in the UK since the government gave cannabis trials the green light two years ago.
Others are checking its effect on MS symptoms and cancer pain.
At the moment patients might be given a powerful painkiller such as morphine for the hours immediately after surgery.
However, doctors are loathe to prescribe drugs like these for too long, so as soon as possible, more conventional painkillers such as codeine or even paracetamol are given.
Reduced dose
Cannabis extracts, it is hoped, might do a better job than these at dealing with pain, and, if used earlier in combination with the morphine, reduce the amount of the more powerful drug which has to be used.
"What we want to do now is prove it does provide pain-relief in a hospital setting.
"We have produced a standardised pill, so we know exactly how much of the drug the patient is getting, and can monitor side-effects precisely."
The cannabis itself is being supplied to the trial by a charity, and the research is being funded by the Medical Resarch Council.
Both a supposed active ingredient of cannabis, called THC, and tablets made from cannabis oil, are being tested.
Patients have mixed feelings about the introduction of cannabis into the hospital pharmacy.
Tom Kelleher, recovering after abdominal surgery, said: "I'm totally against drugs - but it's good to use them in the proper sense, in hospital."
The results of the trials will influence any decision by the government to licence cannabis -based drugs.