People turning to apple cider vinegar tonics as a homeopathic remedy to kick a cold or lose weight might want to consult a doctor before making it part of their routine.
Apple cider vinegar beverages, often called fire cider or switchel, date to the Colonial era when the drinks were created to replace nonpotable water. For decades, the beverage faded from view, but more recently it has been popularized by celebrities like Katy Perry and natural health proponents, who see the tonic as a trendy cure-all for everything from acne to hangovers.
From sparkling switchel to probiotic tonics, brands like Bragg, CideRoad and Pepsi's KeVita, among others, have jumped in and offered up their own take on the drink.
"Homeopathic types of remedies probably have some merit," said Ken Harris, managing partner at Cadent Consulting Group. He noted that the beverages are part of an effort to "contemporize home remedies" and appeal to a consumer who is seeking healthier alternatives.
But the acidic treatment could do more harm than good, said Dr. Fatima Cody Stanford, an obesity medicine physician at Massachusetts General Hospital and Harvard Medical School.
"There's no regulated formulation," Stanford told CNBC. She said if the cider is not diluted properly, tooth erosion and ulcers in the stomach, esophagus and intestines can occur, not to mention burning of skin.
However, the majority of the beverages on the market contain only one to two tablespoons of apple cider vinegar. It is usually mixed with water and sweeteners or spices to make the drink more palatable. Stanford's own apple cider vinegar recipe, which she uses as an energy booster, is usually diluted in at least 8 ounces of warm water.
New Jersey-based CideRoad's switchel contains about a tablespoon of apple cider vinegar in its 14-ounce bottles and is mixed with ginger, water and natural sweeteners like maple syrup and cane sugar, according to the company's founder and CEO, Kevin Duffy.
Duffy told CNBC he doesn't make any claims that the beverage is a health tonic, but he does tout it as "delicious" and "refreshing."
Pepsi's KeVita's Apple Cider Vinegar Tonic, on the other hand, is advertised as a "cleansing probiotic" that helps support digestive and immune health. Each bottle contains four strains of live probiotics, according to the company.
Stanford said there haven't been any very "robust" studies conducted on the effects of apple cider vinegar on humans. The majority of tests, which are very few in number, have been on mice and rats.
Researchers found that apple cider vinegar helped protect the animals from high-fat diets, improved their blood-sugar levels and had cholesterol benefits, but these results don't always replicate themselves in a human study, Stanford said.
Despite the lack of empirical evidence of its benefits, apple cider vinegar's popularity is growing, with many of its fans citing health benefits for their use.
"I have been learning of some of the benefits from colleagues of mine who are nutritionists and in the health and fitness industry and they explained that it promotes food digestion and nutrient absorption," Wade Swikle, a business owner and crossfit athlete in Gainesville, Florida, told CNBC. He said he also uses apple cider vinegar he thinks it will curb insulin production and fat storage.
"It is important to me to stay lean and absorb nutrients so I can function at high energy levels throughout the day," he said.
Swikle was drinking the apple cider vinegar straight, but it burned his throat and caused indigestion. Now, he dilutes a tablespoon of apple cider vinegar into 8 ounces of water without any additional sweeteners.
"The taste is very strong and sour, but as I have started diluting it, it has grown on me a bit," Swikle said.
For many devotees, it seems that taste is secondary to what they think are the benefits.
"I don't enjoy the taste, I just drink it to clear the sinuses during allergy season," said Gina B., a medical student in Washington who requested that her last name be withheld from publication. She said she has since stopped using her own concoction of vinegar, water and honey because it upset her stomach.
"I decided a little sniffly was better than that," she said.
Others have grown accustomed to the taste and continue to use it as a remedy because they see it as effective for their lifestyle.
"I've been using it for about eight years now as needed and the taste just sort of grew on me," said Colleen Reilly, an English teacher in Roanoke, Virginia. She uses it for sinus infections.
The beverage appears to be catching on. CideRoad's Duffy said the company did about $1 million in sales in 2016. The company's sales grew more than 226 percent between 2015 and 2016, according to SPINS data provided by CideRoad.
KeVita's drinkable vinegars have seen growth of 80 percent year-over-year, Andrew Thomas, director of marketing for KeVita, told CNBC.
"We've had a partnership with PepsiCo for over three years, which has allowed us to expand our distribution and provide people across the country access to our delicious and better-for-you drinks," Thomas said. "The acquisition has not only taken our distribution opportunities to the next level but also provides us access to more resources to drive awareness and trial of our products."
While sales of carbonated soft drinks have remained flat in recent years, sales of niche beverages have grown significantly, according to Gary Hemphill, managing director and COO, of Beverage Marketing Corporation research.
"Niche usually stays niche," he said, adding that it's likely these beverages will appeal to a very specific subset of consumers and will not become mainstream. However, these consumers tend to be exceptionally loyal to brands.
So, if you're going to reach for an apple cider vinegar beverage, just heed Stanford's advice and drink in moderation.