Sir Isaac Newton's third law of physics also applies to politics and medicine. The axiom states: "For every action, there is an equal and opposite reaction."
Lawmakers have considered action to require a doctor's prescription to obtain cold medicines containing pseudoephedrine, an ingredient needed to make methamphetamines - a dangerous, deadly and illegal drug commonly known as meth.
Reaction from opponents contends cold sufferers seeking relief do not deserve to be penalized with added costs and inconvenience.
A recent wave of opposition has taken the form of a "Stop Meth Not Meds" campaign sponsored by the Consumer Healthcare Products Association.
Proponents of the prescription requirement - who failed to achieve statewide legislation during the session that ended Friday - have moved their campaign to municipal battlefields.
And they have won victories in about 60 Missouri communities, including a number of towns in Southeast Missouri.
The Southeast Missourian newspaper in Cape Girardeau examined data and reported in a February story: "Statistics kept by the Missouri State Highway Patrol indicate that methamphetamine-related incidents have gone down in some parts of Southeast Missouri since many cities began requiring a prescription to purchase pseudoephedrine."
The evidence deserves consideration.
But statistics from another source - Missouri's electronic database - also must be weighed.
Since 2011, sales of pseudoephedrine in Missouri have been tracked at the point of sale behind pharmacy counters.
According to the National Association of Drug Diversion Investigators, the tracking system has proved "extremely effective" in the battle against meth.
The electronic technology is credited with blocking the sale of more than 49,000 boxes of medicines containing pseudoephedrine in 2011 and another 11,000 in the first quarter of this year.
Not only does electronic tracking "block unlawful pseudoephedrine sales right at the counter, it also ensures that law-abiding Missourians are not hindered from accessing the safe and reliable medicines they depend on to treat common cold and seasonal allergy symptoms," said Charles F. Cichon, executive director of the association.
We agree.
Cold sufferers do not deserve to become collateral victims in the war on meth, particularly not while an existing electronic weapon is proving effective in diminishing production of this deadly drug.