Smoking more than doubles the risk of unexplained strokes among younger adults, a new study warns.

And heavy smoking is even worse, with stroke risk more than quadrupled among adults who smoke more than a pack of cigarettes every day for 20 years compared to people who don’t smoke, researchers found.

“Our findings suggest that continued public health efforts around preventing smoking, especially heavy smoking, may be an important way to help reduce the number of strokes happening to young people,” lead researcher Phillip Ferdinand with Keele University in the U.K said in a news release.

For the study, researchers focused on a type of stroke called cryptogenic stroke. These strokes are caused by a blockage of blood flow, but it’s not clear what caused the clog.

“While smoking has long been linked to ischemic stroke, little is known about how smoking affects people under 50, especially in those with unexplained stroke,” Ferdinand said.

His team compared the medical records of 546 people aged 18 to 49 who suffered an unexplained stroke, to 546 people of matched age and sex who hadn’t had a stroke.

About 33% of smokers had suffered an unexplained stroke, compared to 15% of non-smokers, researchers found.

Gender, age and smoking intensity all played a role in stroke risk among younger adults, results show.

Male smokers had more than three times the stroke risk of non-smokers, and 45- to 49-year-olds nearly five times the risk.

And smoking intensity only caused their risk to skyrocket.

Men who smoked more than a pack a day for 20 years had nearly seven times the risk, and people 45 to 49 nearly five times the risk.

Overall, people who smoked 11 to 20 cigarettes per day had a 64% increased risk of stroke, compared to a 5.2 times risk with more than 40 smokes a day, results show.

"Recent research has shown an increase in younger people having these unexplained strokes, so it is important to evaluate any potential links. Our study found that smoking may be a key factor,” Ferdinand said.

There are many ways tobacco smoke can increase stroke risk, according to the World Stroke Organization.

Cigarette smoke contains thousands of harmful chemicals that can affect the circulatory system, the organization said. It also contains carbon monoxide, which lowers blood oxygen levels, and nicotine, which raises blood pressure.

Smoking also is known to influence cholesterol levels and increase the risk of hardened arteries, the organization added.

The new study appears in the journal Neurology.