CANNABIS has doubled in strength in 11 years, according to a Europe-wide study.

Concentrations of tetrahydrocannabinol (THC), the main psychoactive constituent, in herbal cannabis increased from five per cent in 2006 to ten in 2016.

In cannabis resin, it rose from eight per cent to 17.

Resin typically also contains cannabidiol (CBD), which may offset THC side-effects such as paranoia and memory loss.

However, new production techniques have raised levels of THC but not CBD.

The Bath University study of data in all 28 EU states is the first to track changes in the drug across Europe, where there are about 24 million users.

Lead author Dr Tom Freeman said: “CBD can make cannabis safer without limiting its effects.

"A rise in THC, but not in CBD, may make it more harmful.”