RE: articles concerning marijuana's effect on the human heart. Note that imaginary scenarios are often appended to the descriptions of the studies. Please read carefully as some of this is the usual "reefer madness" propaganda masquerading as science.
In July 2020, it may be becoming legal for researchers in the U.S. to use high quality, high potency marijuana obtained from state-legal dispensaries, for their studies. So far, the U.S. government has only allowed low-grade marijuana produced at the government's own research facility at the University of Mississippi to be used by researchers under U.S. jurisdiction.
This means that all the research conducted in the USA has, so far, used fairly high doses of marijuana, since the marijuana being used was very low potency.
This is in contrast to the "legal marijuana" industry in the USA over the past 19 years or so, that has always mostly sold extremely high potency weed, which means the users would need to use very small amounts to obtain the desired effect. But high potency weed could not legally be used for research under U.S. jurisdiction since the government's own production facility did not produce any high potency marijuana.
Here's a link to information about the type of marijuana required to be used in all studies conducted inside the USA: click here.
When large amounts of low-potency weed are consumed, the ratio of dangerous carcinogens, carbon monoxide, and other toxins, are greatly increased. This is probably the main reason why some of the studies conducted in the U.S. indicate problems for the heart.
If high potency, state-legal weed had been used for the studies, resulting in much smaller amounts being consumed, the results would not have been so negative; in fact, the results might have been the precise opposite.
If the measure approved by the U.S. House making it legal for researchers in the U.S. to use state-legal marijuana from licensed dispensaries, actually becomes law, then the type of weed actually being used legally in the USA, could actually legally be studied. Then we might know more about the effects of such high potency weed on the human heart and so forth.
But there are many other known factors for heart trouble, such as tobacco, that are much better established as being the cause, than cannabis. In the U.S., most marijuana users do not use tobacco with their marijuana, unlike Europeans. The media has repeatedly reported over the years that tobacco has been banned in Dutch marijuana coffee shops. But I'm not sure I believe that. I have not been in the Netherlands since 2009.