Is Obamacare constitutional?

if the BROADLY EXPANDED federal power that the modern Court's interpretations of the Constitution's "commerce clause" gave to the feds, and which underpins *all* of the federal authority to prohibit marijuana sales in the individual States continues to be upheld by the Supreme Court... then the answer to your question about 'is the health care bill constitutional' is probably going to have to be "yes"

Government's authority to ban the purchase of a product (marijuana) is very different from government's authority to force the purchase of a product (insurance).

Not according to the Supreme Court... Both policies find the entirety of their legitimacy in the courts massive expansion of the "commerce clause"...