Dear Doctor: The Food and Drug Administration recently said parents shouldn't give their kids medicine with codeine or tramadol. Just how dangerous are these drugs? And what should parents use instead?

Dear Reader: Codeine and tramadol are medications that are widely used to treat pain. However, not everyone reacts to these drugs the same way. Some individuals metabolize codeine and tramadol very quickly, which results in unsafe levels of the drugs in the body. This puts them at risk of breathing problems severe enough that they can lead to death. As a result, the FDA amplified an existing warning in April regarding the use of tramadol and codeine when treating children.

In its announcement, the FDA said that children younger than 12 should not be given medications that contain either codeine or tramadol. This warning also extends to nursing mothers, who risk transmitting unsafe amounts of the drugs through their breast milk. The FDA also states that tramadol is not to be used to treat pain from surgeries to remove tonsils or adenoids in children under 18 years old.

Manufacturers of drugs that contain codeine or tramadol are now required to change their labels to include a plainly stated warning against their use in anyone under the age of 12, and by nursing mothers. An additional warning cautions against giving codeine or tramadol to adolescents between the ages of 12 and 18 who are obese, or who have sleep apnea, lung disease or any other condition that can affect breathing.

So what exactly are codeine and tramadol, and why do they pose dangers to some younger patients?

Both drugs belong to a class of medications called opiate analgesics. Codeine is approved to treat pain and, because it also belongs to a class of medications called antitussives, it is used to treat cough. Antitussives don't cure a cough, but rather suppress the command from the brain that tells the body to cough. Codeine and tramadol reduce pain in a similar fashion, by acting on neural pathways.

As we mentioned earlier, the problems arise in certain individuals whose bodies absorb and metabolize the drugs very quickly. An enzyme variant in their livers causes them to convert the medications so rapidly that they wind up with unsafe levels of active opioids in their bloodstream. This affects the part of the brain that regulates breathing and can lead to death.

The new warning comes thanks to data collected by the FDA, which showed that some children who were given codeine or tramadol became gravely ill after just a single dose of the medication. The warning to nursing mothers arises from the report of an infant who died after being exposed to codeine in his mother's milk.

As for what you should use instead, in most cases, over-the-counter medications like Tylenol or ibuprofen should do the trick. If not, check with your family doctor to discuss and craft a pain management plan that is both effective and -- here's the really important part -- safe for your child.

(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: My teenager drinks a lot -- I mean, a LOT -- of soda and energy drinks. But I recently read that a teenager died after only three caffeinated drinks. How can this be? How much is too much?

Dear Reader: I understand your concern. The increase in caffeine consumption in our society coincides with the frenetic pace of modern life. Not too long ago, the majority of Americans caffeinated themselves with a cup of coffee at a diner, coffee house, home or work. Now, we have national chains of coffee shops; iced coffees and teas in every grocery store, drugstore and gas station; and an array of caffeinated sodas lining the aisles of supermarkets.

As if the preponderance of coffees and sodas weren't enough, energy drinks have rapidly permeated our culture. Not only do they contain caffeine, they include stimulants such as taurine, guarana and ginseng, as well as large amounts of sugar. These energy drinks are marketed to young adults, especially men, to help general performance. A 2010 survey of U.S. troops in Afghanistan found that 44.8 percent drank at least one energy drink per day. Within the U.S., an estimated 31 percent of those ages 12 to 17 consume energy drinks regularly, with 5 percent of high school students drinking at least one energy drink per day.

Some people rightfully point out that coffee has been linked to health benefits, specifically to a reduction of heart attacks and strokes. But that data isn't as clear-cut as you might think. A 2014 review of 36 studies showed a 5 to 15 percent reduction in the rates of cardiovascular disease among coffee drinkers compared with people who drank no coffee. (Note that one of the authors received a grant from Nestea, part of the Nestle Company.)

Then there was a 2009 study that followed 83,076 nurses and their caffeine intake; it found a 19 percent reduction in stroke among the nurses who drank two to three cups of coffee per day and a 20 percent reduction among the nurses who drank four or more cups of coffee per day.

However, a 2013 study that followed 43,727 men and women for 17 years found an increase in death rates among those who drank four or more cups of coffee per day, and a 21 percent increase in death rates among men who drank four or more cups of coffee per day. Looking further into the data, the authors found that the increase in death rates was seen only for women and men under age 55 who drank coffee, not in those over 55 who drank coffee. In fact, men younger than 55 who drank four or more cups of coffee per day had a 56 percent increase in the death rate.

All told, it's possible that caffeine has a particularly negative effect upon younger people and that the studies in older adults didn't identify this increased death rate in younger people. It's also possible that older adults who drink caffeine consistently are heartier than their younger counterparts who die for reasons unknown.

This brings us back to teenagers who consume sodas, coffees and energy drinks. There are no good studies for this age range, just the unfortunate case reports of teenagers consuming too many energy drinks and developing life-threatening heart rhythms.

My feeling is that moderation can easily lead to excess within our society, and the best way to deal with this is to describe not only the theoretical risk of sudden death with binge drinking, but the obesity and diabetes that sugary drinks can create. I would recommend against buying these drinks for the home -- and encouraging your teenager to use only one of these types of drinks per day.

(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: I read that hospitals in the United States might run out of yellow fever vaccine soon. I'm a natural worrier, but I can't help wondering if yellow fever really is an issue in this country.

Dear Reader: It has been more than a century since the last major outbreak of yellow fever in the U.S. But with several recent new outbreaks throughout the world, including one in Brazil, the supply of yellow fever vaccine is running low at a difficult time.

To understand why the resurgence of yellow fever has health professionals concerned, let's take a closer look at the disease.

Yellow fever is a flulike illness spread by certain species of mosquitoes that are infected with the virus. It affects both monkeys and humans, and is carried between the species by the infected insects.

Not everyone who contracts the virus develops symptoms, which can range from mild to severe. In less-serious cases, patients experience fever, chills, headache, muscle aches, nausea and vomiting. Jaundice, which causes the skin and whites of the eyes to appear yellow and can also be a symptom, is how the disease got its name.

In the small proportion of people who go on to develop a severe form of yellow fever -- about 15 percent of those who are infected -- the disease can cause uncontrolled bleeding, and result in heart, kidney and liver problems. According to the World Health Organization, half of the severe cases of yellow fever are fatal.

The disease, which is believed to have originated in Africa, became an international problem in the 1600s, when shipping began to go global. The infected mosquitoes were transported throughout the world, and yellow fever epidemics began to occur.

During the Spanish-American War, five times as many American soldiers died of yellow fever as did in battle. This led to the formation of a commission that proved what had been suspected since the mid-1800s -- that the disease is spread by mosquitoes. Subsequent sanitation programs, followed by the development of a pair of vaccines in the 1930s, led to the near-eradication of the disease in many areas. The last outbreak in the U.S. occurred in New Orleans in 1905.

Now, because of an increase in disease among monkeys, flagging vaccination programs in some countries, and the mosquito-friendly environment caused by a warming planet, infected mosquitoes are on the move again. This has caused a spike in demand for the vaccine. Prior to 2000, about 5 million doses per year were adequate. By 2007, demand rose to 34 million doses of the vaccine.

American health officials, who say the outbreak in Brazil is uncomfortably close to home, are working to expand the supply of vaccine. Sanofi Pasteur, the main vaccine producer, announced plans to open a new manufacturing facility in the U.S. next year.

In the meantime, federal health agencies are working to make available an alternate vaccine that is imported from France. Clinic sites with this vaccine are listed here: wwwnc.cdc.gov/travel/yellow-fever-vaccination-clinics/search.

(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.)