Dear Doctor: Before I had a double hip replacement at 47, I was an avid daily runner, participating in several marathons per year and any charity event possible. Everything I've read says not to run with hip replacements. Do I really have to cut running out of my life?

Dear Reader: The advice regarding running and hip replacements relies on basic physics. The more stress you put on the new prosthetic joint, the more wear-and-tear can occur, and the sooner you may be a candidate for revision surgery to replace the device. And although the current wisdom is that a modest, even moderate, amount of running can be OK in some cases, the life you have described, with as many races as possible (and, we imagine, plenty of training in between) is not a good fit for someone with artificial hip joints.

In a total hip replacement, the surgeon removes the damaged or diseased bone and cartilage and replaces them with a prosthetic joint. People consider the surgery when medication, lifestyle changes or walking supports fail to have an effect on pain or impaired mobility. From its introduction in 1960, hip replacement has undergone a sustained evolution in surgical techniques, as well as the materials and technologies that go into making the artificial ball-and-socket joint.

However, the data regarding the effects of regular running after a total hip replacement haven't kept pace.

A few studies have looked into what happens when individuals with a total hip replacement continue their running regimens. The results suggested that running could be OK, but there is disagreement among scientists as to whether the sample sizes in those studies were adequate, and whether the subjects were followed for a long enough period of time.

A larger study published several years ago, in which hip replacement patients took part in a diverse range of physical activities, including running, uncovered some potential problems. Though not widespread among the participants, these included fractures, dislocation, loosening of the prosthesis and mechanical wear that resulted in the production of metal ions, which passed into the bloodstream. The long-term effects of this type of exposure are not yet understood.

But here's the good news. Although running isn't recommended for people with total hip replacements who would be new to the sport, it's a different story for experienced runners. Those individuals who had been regular runners before the surgery and who have completed their rehab period can take part in a moderate amount of running. Needless to say, there are some caveats.

-- Before you run a single step, get the OK from your surgeon and your physical therapist. This is non-negotiable. You've put a lot of effort, time and money into your new hips.

-- Take your time and get back into shape with low-impact exercises like swimming and stationary cycling.

-- Be aware of your stride. Twisting of any kind can cause the ball of the prosthesis to separate from the hip bone. Choose flat, straight surfaces that won't tax the joints.

-- Stick to the distance and frequency that your medical team allows.

(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: Not too long ago I developed symptoms of peripheral neuropathy that occurred almost immediately after having an MRI with gadolinium contrast. A physician confirmed that I had retained gadolinium. Shouldn't people be made aware of this potential adverse reaction to gadolinium?

Dear Reader: In short, yes. But before I explain further, let me provide some context: Magnetic resonance imaging (MRI) is an excellent diagnostic tool that doesn't subject a patient to radiation. It's especially helpful for the imaging of the brain and joints, but can be used for any part of the body. Sometimes, patients are injected with a solution -- called a contrast agent -- that enhances the image; most of these solutions contain the element gadolinium. Used for years, gadolinium contrast agents have been thought to be generally safe.

The gadolinium in these solutions is bound to a compound that helps the body excrete it effectively. But people with kidney dysfunction are less able to excrete gadolinium, so their exposure is more prolonged. Gadolinium can then become deposited in the tissues of the body, leading to an inflammatory reaction that causes the abnormal formation of scar tissue, a process called nephrogenic systemic fibrosis, or NSF.

The most notable effects are to the skin, with swelling and thickening occurring two to four weeks after exposure to gadolinium. Symptoms start on the ankles, feet, wrists and hands and, over time, extend upward to the thighs and upper arms. The skin can become so thickened that it affects the movement of the joints, leading to poor mobility of the fingers, wrists, ankles, elbows and knees. People affected may have difficulty using their arms and legs for many daily activities, with some needing a wheelchair for assistance. When severe, NSF also can cause scar tissue to develop in the muscles, heart and lungs. Diagnosis requires a biopsy of the affected skin.

You mentioned that you had peripheral neuropathy related to gadolinium. This has not been widely reported, but sounds theoretically possible for two reasons. First, gadolinium disrupts calcium ion passage in nerve cells, potentially limiting sensation. Second, the swelling and thickening of the skin that happens with NSF may damage the smaller nerves that provide sensation to the skin.

Overall, the risk of NSF with gadolinium in people with kidney dysfunction is 2.5 to 5 percent. To prevent a negative reaction to gadolinium contrast, doctors can assess kidney function beforehand. If the kidney function is low, then an MRI without contrast or with traditional iodine contrast (which can cause kidney problems in itself) may be options. Doctors can also assess the type of gadolinium contrast agent; those containing more unbound gadolinium are linked to greater episodes of NSF.

Lastly, if a patient has kidney failure and is on dialysis, then he or she should have dialysis immediately after the exposure to gadolinium in order to remove the element from the body.

Possible treatments for NSF include phototherapy, plasmapheresis and photopheresis, but these treatments have not yet been proven effective. Cases of NSF with gadolinium peaked in 2010 with 500 cases and have declined since with greater awareness of gadolinium's role in the condition. Thank you for your part in helping raise awareness.

(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've read stories about Girl Scouts selling hundreds of boxes of cookies outside pot shops to people who are anticipating the munchies. My question is: What is it about cannabis that causes that kind of hunger?

Dear Reader: It's a toss-up which effect of using cannabis is better known -- the fact that it alters mood and perception, or the raging hunger that users experience, known as the munchies. Research into the psychoactive effects and mechanisms of the drug dates back decades. Now, thanks to research published in 2015 by scientists at Yale University, we finally have an answer to the munchie conundrum.

According to the World Health Organization, cannabis is the most widely used and trafficked illicit drug in the world. In the United States, it is recognized as a Schedule I controlled substance. However, growing numbers of states are passing laws that permit medical and/or recreational use of the drug, which is how some enterprising Girl Scouts came to set up shop outside cannabis dispensaries.

Cannabis looks and grows like a weed, but it's actually quite complex. Researchers have isolated more than 400 different chemical compounds and entities within the plant, a number of which are being studied for medicinal qualities. In fact, the mechanisms revealed in the research that decoded the munchies show promise in helping people who need help with appetite, such as cancer patients undergoing chemotherapy.

The active ingredient in cannabis is tetrahydrocannabinol, or THC. It is chemically similar to anandamide, a neurotransmitter that naturally occurs in our brains. In fact, the molecular structures of THC and anandamide are so similar that THC can attach to and activate sites known as cannabinoid receptors, which are located on neurons throughout the brain. Once THC enters the blood, either by smoking or ingesting, it takes just a short time for it to bind to receptors and begin to affect a range of cognitive processes in the brain.

In 2014, a team of European researchers discovered that THC fits into a neural structure in the brain known as the olfactory bulb. The result is an increased sense of smell, which the scientists believed could lead to an increase in appetite. A year later, a team of neuroscientists from the Yale School of Medicine published a surprising set of findings that focused on the drug's effect on the appetite centers of the brain.

Working with genetically modified mice, the scientists found that some of the compounds in cannabis actually trick the appetite center of the brain. Through a series of complex steps, those compounds subvert the very mechanism that, when working properly, tells the mice that it's time to stop eating. As the lead researcher of the study put it in a press release, "It's like pressing a car's brakes and accelerating instead."

Scientists are intrigued by this newest study. A compound that flips a biochemical switch and makes a brain process do the opposite of what was intended is indeed unique. However, the consensus is that this, like that olfactory bulb discovery, is just one piece of the complex munchies mystery.

(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.)