As the campaign to legalize medical marijuana in Florida heats up, so too does an enduring worry about whether an herb intended for sick adults will end up harming teens.

Evidence is mounting that heavy pot use among young teens can cause serious emotional and physical damage. The fear is that medical marijuana will boost pot consumption by making it more abundant and socially acceptable.

This is one of medical marijuana's hidden dangers, opponents often say.

Surveys of teen pot use, however, tend to dispel the theory. The research is complicated, but so far, the numbers are reassuring.

America's experiment with medical marijuana began nearly two decades ago. Twenty states and the District of Columbia have now approved it. Florida voters will decide in November whether to hop on board.

Yet reported pot use among teenagers has barely changed since 1996, when California broke the medical marijuana ice and became the first state to legalize its use.

Kids do smoke more pot in the West and New England — where medical marijuana abounds — but that was true before the laws were passed.

Most states, whether they allow medical marijuana or not, follow broad national trends. Teen usage might rise for a while in a medical marijuana state like Oregon but it also happens in conservative Texas. Usage may drop in Alabama, but also in Colorado, a marijuana mecca.

As more states have joined the ranks, reported teen use has remained essentially flat in recent years.

"We cannot draw a clean link between medical marijuana and shifting drug usage rates,'' said Dr. Wilson Compton, deputy director of the National Institute on Drug Abuse.

Surveys can be unclear because they rely on kids to honestly report their usage. If the numbers rise, it could mean teens are smoking more or that maybe they are just more comfortable admitting it. The same goes for interpreting declines in usage.

It also can be years after passage of a medical marijuana law before its impact is fully felt, which also clouds attempts to measure behavioral change.

"We have a tendency to look at all medical marijuana states in a single category,'' Compton said. "Yet they have different timing, different implementation systems, sales and distribution. There are huge distinctions.''

Compton and others worry that medical pot laws might harm some teens, even if overall usage is not altered. A few studies, for example, have reported rising drug arrests and mental health admissions in medical marijuana states.

As for overall teen pot use, the medical marijuana era offers only one solid lesson for Florida voters: Kids choose intoxicants for many reasons that have little to do with adult laws.

Use goes up and down

The brains of young teens are still developing. Scans of heavy pot users show "a profound decrease in connectivity'' between brain cells in certain areas, said NIDA director Nora Volkow. Particularly vulnerable is the hippocampus, "which is important to how we memorize and learn and control emotions.''

According to a University of Michigan survey last year, one-fifth of high school seniors said they had smoked pot in the previous 30 days, and one in 16 smoked daily. By comparison, two-fifths of seniors said they had drunk alcohol in the previous month; one-fifth say they binge drink.

The annual survey, called Monitoring the Future, shows dramatic swings in pot use well before medical marijuana came on the scene. In the late 1970s, nearly 40 percent of high school seniors said they smoked marijuana at least once a month, but by the early 1990s, only about 10 percent did. Then, over just a few years, usage doubled.

A common theory for these swings holds that parental and societal attitudes play a role. If pot use seems to rise steadily, parents and politicians may crank up antidrug messages and allocate money for intervention. When use wanes, society relaxes, only to see another upswing.

Opponents say medical marijuana can boost use by sending a message that pot isn't so bad.

Evidence of the theory, though, is hard to come by.

Ten states legalized medical marijuana between 1996 and 2004, but meanwhile monthly usage by high school seniors dropped 4 points, to 18 percent.

Then usage headed back up for a while, then it flattened out.

Eighteen states and the District of Columbia now have medical marijuana programs up and running, and two more approved it last year. Yet from about 2010 on, "there is no significant difference'' in teen pot use, NIDA's Compton said. "The numbers are the same, virtually identical.''

The devil, the details

University of Florida professor Sarah Lynne-Landsman found no measurable connection between medical marijuana and teen use in a 2013 American Journal of Public Health report.

She thinks medical pot can ease suffering, such as when a patient with wasting syndrome battles nausea. "You can't ask them to take oral meds if they are throwing up everything.''

But Lynne-Landsman urges caution if Florida sets up a medical marijuana system, because although there appears to be no correlation with teen use, further research might turn up new information.

"We don't have enough information to say for sure that these laws will not result in problems for youth,'' she said.

Florida's proposed constitutional amendment would allow sales only through licensed dispensaries. A 2013 study by RAND researchers reported an association between dispensaries and higher teen pot use. But the possible link seemed stronger in the few Western states that allowed dispensaries early on, and weaker in states that established dispensary systems more recently.

"Even though nationwide we are not seeing huge increases'' in teen use, Lynne-Landsman said, "we are going to have to strictly regulate things. The devil is in the details.''

To avoid a new version of the pill mills that doled out huge quantities of prescription painkillers, a registry should keep track of which doctors are approving pot and for what reasons, she said. Dispensaries should be limited in number and kept away from schools and churches.

"We don't know what these changes in policies are going to do,'' Lynne-Landsman said. "It's better to err on the side of caution.''

Stephen Nohlgren can be contacted at nohlgren@tampabay.com.