The first investors in GW Pharma more than two decades ago were a little confused to be invited to visit a greenhouse in Kent. One of them even recalled having to wash sticky cannabis residue off his hands afterwards. In 1998, the therapeutic drugs start-up had little more than a crop of weed and a good idea.

“It was certainly an uphill battle,” admitted chief executive Justin Gover. “We were, after all, dealing with a plant that was highly controversial.”

Today, GW Pharma is worth $4.5bn (£3.5bn) and has won approval for two drugs — Sativex, for multiple sclerosis, and Epidiolex, for epilepsy. If it had been launched today, it would have enjoyed a different response to its fundraising.

Entrepreneurs and investors are rushing into