GW Pharmaceuticals, the firm founded to make medicines from cannabis, warned yesterday that government approval for its first product, a pain-killing mouth spray, had been delayed for a second time.

Sativex, which it hopes to market to sufferers of multiple sclerosis, will not win a regulatory thumbs-up until at least the second half of the year, GW said, causing its share price to slide by more than 20%.

GW had originally hoped to get Sativex approved by the end of last year, and then set this June as a deadline. Geoffrey Guy, GW's executive chairman, said yesterday he was still confident it would be approved but refused to set a new target date. "I've already got it wrong twice," he said. GW's shares hit a 16-month low of 139.5p after the news.

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