By Drs. Mehmet Oz and Michael Roizen

"I ain't left this little room … tryna concentrate to breathe, cause this piff [weed] so potent, killing serotonin," sings Grammy-winning R&B singer The Weeknd in his song "Initiation." With over 4 million views on YouTube, it seems the mood-enhancing neurotransmitter and all-around gut-loving hormone serotonin has made its way into mainstream pop culture! The Weeknd is probably right; research shows that chemicals in marijuana lower serotonin levels. The repercussions are far more serious than the munchies.

For a new study, Johns Hopkins researchers reviewed brain scans of over 50 people, and found that those with mild cognitive decline had lower levels of serotonin compared with people with healthy brains. Maybe protecting serotonin levels (bye-bye, weed) will help you avoid progression to Alzheimer's disease.

Another lab study found that a lack of neurons that modulate serotonin can lead to a buildup of amyloid plaques in the brain — one of the hallmarks of Alzheimer's. Decreased respiratory and cardio health (caused by smoking anything) may contribute to decline of healthy neurons.

While scientists untangle the link between Alzheimer's and low serotonin, you want to make sure enough of this chemical is circulating in your brain and gut. Serotonin modulates a whole host of things, from mood and appetite to sleep and sexual function. Boost your supply by:

• Shooting for at least 30 minutes exercise daily (more is better).

• Catching some rays. Exposure to bright light of any kind can boost serotonin levels.

• Eating foods with tryptophan (turkey, nuts, pumpkin seeds and salmon).

When kids take water into their lungs — beware and be aware!

You know how, right after you jog or do free weights, you feel fine, but the next day your muscles are tender and sore? This phenomenon was first described by Dr. Theodore Hough in 1902. He called it delayed-onset muscle soreness, or DOMS. But today, 115 years later, scientists are still debating why it happens.

Well, dangerous and severe discomfort can set in after swimming, too. The conditions are called atypical or dry drowning, and secondary drowning — but we know exactly what causes them.

Atypical drowning happens (mostly in kids) when you take in water while swimming. Such a near-drowning experience triggers laryngospasm — constriction of airway muscles — and that deprives the body of oxygen. Worse, when you try to breathe, that suction disrupts the junctions between cells in the lungs, triggering edema and making the lack of oxygen even harder to correct. "That's why every child who's fallen into the water or experienced a near-drowning should be taken to the emergency room immediately," says Dr. Purva Grover, medical director of Cleveland Clinic's Children's Pediatric Emergency Departments.

Secondary drowning occurs when someone has gotten water into his or her lungs (again, usually a child) without being aware of it. If it causes pulmonary edema, within an hour there's rapid or difficult breathing; however, sometimes symptoms don't show up for 24 hours. Then they can include respiratory problems plus vomiting, lethargy and a lack of desire to eat or drink.

Seek medical attention immediately if your child experiences any of these symptoms after being in or around water.

Singing with Parkinson's

When Michael J. Fox rocked out on "Johnny B. Goode" with Coldplay during a concert at New Jersey's MetLife Stadium in July 2016, it was wonderful to hear him make liquid magic with his guitar solo — just like he did in the 1985 movie "Back to the Future."

Fox was diagnosed with Parkinson's disease in 1991, and it appears that he may have tapped into a secret therapy for that neurological condition. It seems making music is a powerful tool for managing symptoms of the disease, which include stiffness and tremor, trouble standing or walking, poor balance and impaired voice, as well as cognitive problems and more.

Recently, Iowa State University researchers looked at how singing can strengthen muscles that control swallowing and breathing — two functions that are damaged by Parkinson's. Their study published in Complementary Therapies in Medicine confirmed that singing, even if off-key, improves those abilities. And researchers in Lancet Neurobiology showed that music-based interventions improve everything from Parkinson's-related motor performance, speech and cognition to brain systems that stimulate feelings of reward and arousal, and affect regulation, learning and agile formation of new memories.

Music has been shown to have powerful health benefits in other situations as well. A steady rhythm helps regulate breathing and, if the tunes are relaxing, they lower levels of stress hormones, such as cortisol. It's also been suggested that music is a great way to ease post-surgery stress and pain. In other words, when it comes to complementary medicine, Music B. Good.

Rules for safely trimming your hair (wherever it grows)

In this year's "War for Planet of the Apes," Andy Serkis (the ape Caesar) and Woody Harrelson (The Colonel) sport virtually the same coiffure. It seems both primate and tightly wound white guy want closely cropped hair.

That impulse, apparently, isn't confined to the head or the silver screen. A study published in the journal JAMA Dermatology reveals that more than 76 percent of 5,600 adults surveyed said they'd groomed their pubic hair. The real shocker: More than a quarter of those folks had been injured while doing it to themselves or having it done to them — and around a third of those surveyed said they'd injured themselves five or more times!

Most injuries aren't serious (minor rashes, cuts or burns), but opening even small nicks and cuts in the nether regions increases your risk for sexually transmitted infections. And more than 3 percent of those surveyed did say that post-shave, they needed to take antibiotics and (no kidding, ouch!) around 2.5 percent needed surgery. So here's our advice. If you've gotta do it:

• Clean the area completely with soap and water before proceeding.

• Use shaving cream or lotion to prep the area.

• Pay attention! No texting, and steer clear of tender areas (some folks are shaving/grooming areas in close proximity to the vagina, anus, scrotum).

• Shave with the grain; use only safety razors; no scissors or straight blades, please! (That might explain the surgeries.)

• If you nick yourself, wash with soap and water; apply alcohol or an antibiotic cream to disinfect the area. Avoid sexual contact until it heals.

DHA omega-3 fish oil boosts brain estrogen

"To catch a fish, think like a fish." That's an aphorism repeated over and over in a New York Times opinion piece, YouTube videos and online seminars for entrepreneurs. We'd add: To think (period), catch some fish oil (DHA omega-3)!

A lab study in Scientific Reports found that taking supplements of DHA -- the omega-3 fatty acid found in fish oil (and algal oil; algae is where the fish get it from!) and synthesized from ALA (alpha-linolenic acid) found in foods such as walnuts and flaxseed -- boosts production of estrogen in the brain. Yes, guys, it's essential, and since you convert only 2 percent of ALA into DHA, getting plenty is vital.

Estrogen is important for brain health: It helps reduce nerve damage from stroke, and protects the brain from environmental toxins such as methyl mercury. And an estrogen imbalance in the brain is linked to changes that contribute to Alzheimer's disease, stroke, depression and some autoimmune disorders.

Benefits of DHA: Studies show (inconclusively, says the National Institute for Complementary and Integrative Medicine) that fish oil decreases tumor cell proliferation. We know that it reduces the effects of hypertension, arthritis, atherosclerosis, depression, diabetes, myocardial infarction and the risk of blood clots. Plus, in a randomized study, people who took 900 mg of DHA omega-3 daily for 28 weeks had improved learning and memory function. We say their brain age was six years younger! So take 900 mg DHA algal oil daily, unless you're on a blood thinner; then check with your doc and stick with salmon and ocean trout.