The findings are noteworthy because cannabis isn't normally associated with acute physical problems, let alone death.
The researchers, based at Duesseldorf and Frankfurt's university hospitals, said these were isolated cases but suggested people who might have serious heart problems should be made aware of the risk of cannabis use.
Their study was published online this month in the journal Forensic Science International.
The cases involved two outwardly healthy young men, aged 23 and 28, who died unexpectedly after smoking cannabis.
"To our knowledge, these are the first cases of suspected fatal cannabis intoxications where full post-mortem investigations (...) were carried out," the researchers said.
Autopsies showed that the younger man had a serious undetected heart problem and the older one had a history of alcohol, amphetamine and cocaine abuse.
These underlying conditions, combined with cannabis's known effect of increasing a user's heart rate or blood pressure when lying down, may have caused the men's hearts to lose their rhythm.
Dr. Benno Hartung, one of the study's authors, said the cases were among 15 deaths the scientists reviewed between 2011 and 2012. In 13 of them, other factors likely were to blame.
"We assume that these are very rare, isolated cases," Hartung said in an email to The Associated Press on Wednesday, noting that it was hard to draw any wider conclusion about the number of deaths that might be caused by cannabis use.
Still, the researchers concluded that while cannabis isn't particularly toxic and its effects are short lived, people who are at high risk for cardiovascular diseases should avoid the drug.
Dr. David Nutt, who is chairman of Britain's Independent Scientific Committee on Drugs and wasn't involved in the study, said the researchers had presented an "exceptionally complete collection of evidence in support of their theory that, unusually, cannabis was the trigger for these two tragedies."
"People with vulnerable hearts should be informed of this risk with cannabis," he said.
Nutt said an additional strain to the heart from strenuous activities can have similarly fatal consequences in people with underlying heart conditions.
By comparison, alcohol and tobacco use are accepted as the reason for a large share of premature deaths in Europe each year.