The FDA has its power to regulate consumer products under two provisions of the Constitution: the general welfare clause (since product safety is directly related to well-being, in that a product defect should not disable, poison, kill, or otherwise harm a person).
Article I, section 8 of the U. S. Constitution grants Congress the power to "lay and collect Taxes, Duties, Imposts, and Excises, to pay the Debts and provide for the common defense and general Welfare of the United States."
...and the commerce clause, since very few products are produced and consumed within a single state and no other.
Article I, Section 8, Clause 3 states that the United States Congress shall have power "To regulate Commerce with foreign Nations, and among the several States, and with the Indian Tribes."
I think that many, if not most, states have product regulatory agencies. All that would happen if the FDA were disbanded as some people advocate is that each state would then regulate each product separately, leading to a plethora of regulations, many of them contradictory and incompatible with each other. The costs of complying with 50 different regulatory systems would cause product prices to skyrocket for any industry that wanted to sell its product across state lines.
Note that none of the Constitutional authority for the FDA to perform regulatory functions according to its charter apply to use of the FDA to try to control behavior. For instance, the FDA can tell a soda company that its product must adhere to certain safety standards, but it cannot control consumer use of the product. It is your choice, as a consumer, to consume that soda. It is the FDA's job to ensure that it is made from clean water and not contaminated with bacteria.
Article I, section 8 of the U. S. Constitution grants Congress the power to "lay and collect Taxes, Duties, Imposts, and Excises, to pay the Debts and provide for the common defense and general Welfare of the United States."
The notion that "provide for the general Welfare is a grant of power has been debunked so thoroughly and so often here I'm surprised you even tried that.
...and the commerce clause, since very few products are produced and consumed within a single state and no other.
Article I, Section 8, Clause 3 states that the United States Congress shall have power "To regulate Commerce with foreign Nations, and among the several States, and with the Indian Tribes."
James Madison had this to say about the Commerce Clause:
"For a like reason, I made no reference to the "power to regulate commerce among the several States." I always foresaw that difficulties might be started in relation to that power which could not be fully explained without recurring to views of it, which, however just, might give birth to specious though unsound objections. Being in the same terms with the power over foreign commerce, the same extent, if taken literally, would belong to it. Yet it is very certain that it grew out of the abuse of the power by the importing States in taxing the non-importing, and was intended as a negative and preventive provision against injustice among the States themselves, rather than as a power to be used for the positive purposes of the General Government, in which alone, however, the remedial power could be lodged."
Given that, if this is a valid exercise of the Commerce Power within it's intended purpose there should be some manner of injustice that the existence of e-cigarettes has or will cause to be visited on one state by another, which banning them will remedy or prevent.
If any such injustice exists, I'm at a loss to see what it is.