He has held a chair in Practical Philosophy at Stockholm University since 2002 and he is Affiliated Professor of Medical Ethics at Karolinska Institute.

His distinctly utilitarian ethical views have made him a controversial figure, notably within the medical establishment and for the Swedish Christian Democrats.

In 2001, he debated analytic philosopher and Christian apologist William Lane Craig on the subject, "If God Is Dead, Is Everything Permitted?".

[4][5] A review of the second volume by conservative historian of ideas Svante Nordin calls Tännsjö's views on abortion, euthanasia and other issues in medical ethics “consistently crazy”.

Nonetheless, Nordin praises Tännsjö's intellectual honesty and integrity, noting that he is “not primarily looking to show that [his] opponents are bad people, but rather that they are wrong.