There are some amount of people pursuing strong caffeine. Previously, I introduced an Australian cafe offering extremely dense coffee.
My past entry: An Australian café offers toxic amount of caffeine
And recently, a product with a similar concept is available also in the US. You can buy "Black Insomnia," as well as "Death Wish."
Amazon.com: Black Insomnia
Amazon.com: Death Wish
But are they safe for the human body? Probably, not. You should be cautious to its adverse effect especially you have a heart disease.
The limit of caffeine in a day you can take is said to 400mg. It is equivalent to four cups of regular coffee. In contrast, Death Wish contains 702 mg of caffeine in a cup.
The Atlantic: Drinking Four Cups of Coffee Is Probably Safe
Caffeine has several effects on the body. Taking much caffeine makes you hypertension. It will be a cause of myocardial infarction, at least theoretically. But, it is highly likely you are safe after taking this extreme coffee. The toxic effect of caffeine is too weak to have a fatal influence on your heart.
Nonetheless, there is a case of fatality due to excessive taking caffeine in Japan recently reported. I never recommend you to take caffeine excessively.
Some people pursues stimulus. It is a kind of addiction. The addictive tendency is often elevated with chronic exposure. If you are indulged in coffee, take a break for a while.