Dear Doctor: One of our teens, who is obsessed with science and medicine, says she read a story that there's something in toothpaste that causes colon cancer. Now she's refusing to brush her teeth. Is there any truth to what she's read? If so, should we all stop using toothpaste?

Dear Reader: Your teen is referring to triclosan, a highly effective antimicrobial and antifungal agent. This won't make her happy, but it's found in more than 2,000 consumer products, including cleansers, personal care and household goods, and some pet supplies. That means it's found in everything from toothpaste, clothing and makeup to kitchenware, furniture and toys.

In 2016, the FDA stopped manufacturers from offering for sale any over-the-counter antiseptic wash products like liquid, foam and gel hand soaps, bar soaps and body washes that contain triclosan. The ruling came as a result of studies that found the compound can alter hormone regulation in animals, could have a hand in the development of antibiotic-resistant germs and might be harmful to the immune system. So far, the FDA hasn't stepped in to prevent the use of triclosan in toothpaste, where it is claimed to help protect against gingivitis. And for products like clothes, cookware, furniture and toys, which don't make health claims, the regulation of triclosan is up to the Environmental Protection Agency.

The reason your teen is talking about triclosan right now is the publicity generated by the results of a recent study, which linked even short-term oral exposure to triclosan with adverse effects on the colon and its colonies of beneficial bacteria. Before we get to the details, it's important to note that these studies were done on mice, and that further research regarding the effects of triclosan on humans is needed. However, because the mouse genome is similar to our own, with many shared genes, research in mice offers insights into certain risk factors that can extend to the human population.

Regarding this new study, mice who were fed a diet laced with triclosan over the course of three weeks wound up with inflammation of their colons. They also had a gut microbiome that was measurably depleted, particularly of Bifidobacterium, a strain that has been shown to fight inflammation. Another group of mice who had triclosan introduced into their diets and were then induced to develop inflammatory bowel disease had symptoms that were more severe, and colon damage that was more extensive, than the non-triclosan mice. Although some triclosan mice developed colon cancer with tumors that were larger and more aggressive than those of the non-triclosan mice, the researchers reported that the differences between the two groups were too small to be statistically reliable.

Because colon cancer is associated with inflammation, interest in these new triclosan studies is quite keen. We fully expect to see more research into the subject. In the meantime, no, we definitely don't think you should stop brushing your teeth. But there are toothpastes without triclosan, and identifying them for the family seems like a great project for the young scientist in your household.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: For years now, I've been trying to eat more fruits and vegetables, less meat and a whole lot of olive oil and mixed nuts. Now I'm seeing that one of the studies supporting this has been retracted. Can I go back to hamburgers and fries?

Dear Reader: The Mediterranean diet has been the focus of all sorts of attention in the decades (yes, decades) since it was first introduced. Based on the eating habits of people living in southern Italy, Spain and Greece in the 1940s and '50s, it's just as you outlined -- a fruit- and vegetable-forward diet with lean proteins, whole grains, and plenty of nuts, legumes and olive oil. First publicized in the mid-1970s, the diet really entered the public consciousness in the 1990s. That's when a spate of studies looking into the eating plan got a lot of ink, and diet books devoted to the concept began to hit best-seller lists.

In 2013, a large study in Spain randomly assigned 7,447 individuals to either the Mediterranean diet or to a low-fat diet. All were overweight, and all had risk factors for heart disease, such as smoking and diabetes. Although the study participants assigned to the Mediterranean diet faithfully followed the plan, those assigned to the low-fat diet tended to return to their normal eating habits. As a result, the study turned out to be as much a referendum on the modern diet as it was about the Mediterranean plan. After five years, researchers concluded that the Mediterranean diet lowered heart disease by 30 percent.

But did it? According to a recent analysis of the study by a scientist in England, it's impossible to know. That's because of two extremely important words in the description of the 2013 study -- randomly assigned. For the results of these types of studies to be free from bias, participants must be randomly assigned to the treatment option they will follow. Without absolute randomization, you can't be confident the resulting data is the effect of the treatment. And as you've probably already guessed, the English scientist discovered that the 2013 study did not, in fact, use absolute randomization. In some cases, entire villages were assigned to the Mediterranean diet. Although those participants all followed the same food rules, they also shared other important environmental factors that could affect their health outcomes. This was sufficient to derail the claim of randomization for the 2013 study, and to merit both a retraction and a rewrite by the New England Journal of Medicine in June.

The original takeaway of the 2013 study was that several thousand people had fewer heart attacks and strokes because they followed the Mediterranean diet. The new conclusion is several thousand people had fewer heart attacks and strokes and they followed the Mediterranean diet. In this study, the diet can no longer be cited as the specific reason for the health benefits.

Finally, regarding your question about burgers and fries, the answer is moderation. As an occasional treat? Probably fine if you're in good health. But as a dietary staple? Please don't.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: How often do flashing lights cause seizures? Does it happen only in people with epilepsy? I ask because I recently saw a warning about flashing lights in the new "Incredibles 2" movie.

Dear Reader: While the idea that the makers of an animated children's film would issue a health alert to potential viewers sounds like an urban legend, that is indeed what happened with "Incredibles 2," the long-awaited sequel to "The Incredibles" from Disney. Specifically, several sequences -- in which the film's villain uses a strobelike weapon -- can act as triggers for a seizure for some people. The issue emerged the same day the film opened as discussions of the specific scenes spread on social media. Several blog posts written by people susceptible to seizures due to strobing lights also helped move the issue into the mainstream. According to one blogger, whose post on the topic went viral, "After last night, I can say that the movie is unlike anything I have ever seen before, in that the villain's weapon of choice can hurt not only characters on screen, but can also hurt the people in the audience as well. The weapon? Continuous sequences of rapidly flashing/strobing lights."

The condition is known as photosensitive epilepsy. It affects about 3 percent of those living with epilepsy, which is a disorder of the brain in which abnormal electrical activity can lead to a range of different types of seizures. In photosensitive epilepsy, lights that flash at certain intensities, in certain visual patterns, or with contrasting light and dark patterns, can act as triggers for a seizure. According to the Epilepsy Foundation, photosensitive epilepsy tends to be more common among children and adolescents, particularly those with generalized epilepsy and a type known as juvenile myoclonic epilepsy.

But other populations can be adversely affected by strobing lights and patterns as well. These include anyone who is prone to migraine or other headache disorders, people with vertigo, individuals with autism or ADHD, and those living with non-epilepsy seizure disorders. For these populations, the sequences of the film with the flashing lights -- one scene reportedly lasts for more than a minute -- may not cause seizures, but can lead to headache, nausea, balance issues, light sensitivity, loss of bladder control or general physical discomfort.

There is no cure for photosensitive epilepsy, and once a seizure or other physical reaction has started, there is no way to stop it. Therefore, it's to Disney's credit that, as soon as the issue with the strobe lights emerged, the studio sent out an advisory to all theaters showing the film. The theaters then posted prominent signs, and some even had their ticket takers issue verbal warnings regarding the potentially disruptive scenes.

If this sounds a bit familiar, it may be because back in 1997, an episode of a "Pokemon" cartoon that featured similarly strobing lights adversely affected nearly 700 children in Japan. Many of them wound up in hospitals, and the issue of photosensitive epilepsy moved from medical books to the front pages of newspapers.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)