The addition of tobacco to a cannabis joint does nothing to the experience of being stoned, experts have said.
Researchers said this dispels the myth that adding tobacco to cannabis makes users more stoned.
The authors of a small new study said smoking tobacco with cannabis "does not improve the subjective effects of cannabis, and makes it more harmful to one's physical health".
Experts from University College London (UCL) set out to assess how the drug and tobacco interact when mixed together in joints - which is how the majority of European users consume cannabis.
Their study, published in the journal Psychological Medicine, examined 24 healthy, non-dependent but experienced users of cannabis and tobacco.
Each took part in four sessions of smoking joints that included cannabis and tobacco, cannabis and a placebo, tobacco and a placebo, or just the placebo of both.
They also undertook assessments for both episodic and spatial memory, had their heart rate and blood pressure monitored and self-reported feelings of being stoned or dizzy.
Consistent with previous studies, the researchers found that cannabis impaired the participants' memory but adding tobacco reduced this impairment.
They found tobacco had no effect on users' experience of being stoned.
"There's a persistent myth that adding tobacco to cannabis will make you more stoned, but we found that actually, it does nothing to improve the subjective experience," said the study's lead author, Chandni Hindocha of UCL's Clinical Psychopharmacology Unit.
"In a previous study, we found that the large majority of cannabis users in Europe smoke cannabis with tobacco.
"Tobacco's ability to reduce the memory-impairing effects of cannabis may be part of why people add it to their joints.
"Surprisingly little research has been done on how tobacco might alter the effects of cannabis. As cannabis gets legalised in more countries, it is essential that any changes in cannabis policy consider their interrelationship."
The researchers also found that heart rate was highest when tobacco was combined with cannabis, while there was a moderate increase in blood pressure among participants who combined the drugs.
Senior author Professor Val Curran, also from UCL, added: "There is a clear public health implication here, suggesting that smoking tobacco with cannabis does not improve the stoned feeling but is still worse for physical health."