Professor Griffith Edwards, of the National Addiction Centre, told the Lords committee on science that it would be reasonable to investigate anecdotal evidence from MS sufferers and others who claimed the drug had helped to ease their symptoms.
He was against going to a full controlled trial under the Medical Research Council, and said that ethical questions would be raised by putting human guinea pigs through trials in blind tests.
''One should have a small series of open clinical investigations. I understand from the Home Office there is no legal bar to such investigations and I would think that a controlled trial is somewhat down the line," he said.
The BMA, which supports the use of cannabis for medicinal purposes, will be giving evidence to the Lords committee on science on 21 April.
But the committee was warned against supporting the legalisation of cannabis because it was fashionable among the "polite middle-class smoking-jacket world" of university campuses.
The professor said that in Camberwell, south London, where many of his patients live, some people consumed quantities of the drug that would be unthinkable among the university social set. "Within a culture where people use drugs for the drug effects you get very heavy and very sustained use of cannabis going on," he said.
Cannabis had been used widely for medicinal purposes in Britain for many years, he said, but it still produced side-effects. In the 1880s, a GP writing in the Lancet reported producing acute psychosis after using cannabis for sciatica.
Casting doubt on the medical use today of cannabis, he said better drugs had been produced such as aspirin and the use of cannabis had naturally wasted away.
Professor Edwards cautioned the committee against sanctioning the use of a drug for medicinal purposes which could be sprinkled on tobacco and abused.
"Without pressing panic buttons, if cannabis prescriptions became widely available you are opening up the potential for misuse," he said.
Professor Edwards revealed he had changed his mind about the dangers of cannabis. "I used to take the view that it would not cause dependency. I would today have to change my mind." There were now trials in the US for clinical dependency and the worrying evidence against cannabis was increasing, he said.
It produced in some users the feeling that trees were moving as they walked past them, or that an arm was "floating off".
A consultant specialist in drug abuse, Dr Morfydd Keen, said there was new evidence suggesting that heavy use could cause long-term "cognitive deficits" in the brain.
The committee is concentrating on the scientific evidence, but it has invited written evidence which may be submitted as e-mail to email@example.com, but a paper copy should be submitted as well. More information is available from the clerk to the committee, Andrew Makower, House of Lords, London SW1A OPW (0171 219 6075, fax 0171 219 6715)Reuse content