Barcelona is planning a pilot scheme to decide whether cannabis can be used to help patients with painful illnesses.
Some cannabis studies have raised concerns about mental illness
There is still disagreement between the Catalan authorities and the Spanish government over whether the pills will be available at chemists for the trial.
Madrid would prefer to see the results of a purely hospital-based trial before sanctioning wider use.
Dozens of Barcelona chemists back the project - set to include patients with cancer, multiple sclerosis and Aids.
The Catalan regional authority - or Generalitat - has said it is ready to start the project later this year, using four hospitals and 60 pharmacies.
But the Spanish Health Ministry told the BBC News website it was still waiting for Barcelona to present the final details of the scheme.
A ministry spokeswoman said she did not think the pharmacies would be taking part yet - not until in-hospital trials had been carried out and evaluated.
The Barcelona Pharmacists' Association, Collegi de Farmaceutics, which proposed the project, says the use of chemists is key.
They say pharmacies allow closer interaction with patients and controlling the use of the cannabis pill would be no more complicated than morphine or methadone.
The association, which offers information leaflets on therapeutic cannabis use to the public, says it is aware that a significant number of patients already use the drug for therapeutic purposes.
"This implies risk, since the absence of medical supervision and the variability of the active drug substances contained in the plant make it very difficult to ensure correct dosing and follow-up of consumption," it says on its website.
A UK study of cannabis-based drugs last year revealed patients using cannabinoid compounds could find relief from some of the painful symptoms of MS.
Doctors in the Netherlands were allowed to start prescribing medicinal cannabis to patients in 2003.
Several studies have also linked the drug to mental health problems, such as depression and schizophrenia, and respiratory illness.