Most of the laws passed by the modern Congress are rooted in the Commerce Clause of the Constitution. The Supreme Court has ruled that the constitutional definition of interstate commerce is very broad: anything that moves between the states, or any business that uses raw materials that come from out of state are considered interstate commerce. For example, a barbecue restaurant whose only customers were local instate residents was found to be involved in interstate commerce because it bought food and condiments that originated outside the state. Although the delivery of medical services to individual patients is a local activity, everything that medical care practitioners use, from drugs to bandages to the paper that the medical records are written on, comes from other states. The courts have repeatedly found medical care businesses to be subject to regulation as interstate commerce.

Although the states have ceded the power to regulate interstate commerce to the federal government, they retained, via their police powers, the right to regulate any activity that poses a threat to the public health or safety of their citizens. Thus, state laws that seek to ban the import of milk from outside the state to protect local dairies are an unconstitutional attempt to control trade. In contrast, state laws that focus on consumer protection, such as requiring the sanitary inspection of imported milk and banning the import of contaminated milk, are constitutional, provided they also apply to milk produced within the state. This distinction between laws affecting commerce and laws affecting health and safety is important to medical care practitioners because the regulation of the practice of medicine is considered a health and safety issue and thus reserved to the states as a police power.