Last month, Lindsey Christianson posted on Facebook that she'd be in various parking lots in the north metro in a gray Town & Country minivan with an "Elderberry Queen" bumper sticker.

When she showed up, dozens of people ­— mostly moms — were eagerly waiting to stock up on Christianson's homemade elderberry syrup, a concoction made from organic berries, spices, raw honey and apple cider vinegar.

"I have five boys who used to be sick every fall and winter," Christianson said. "I started making the syrup and they stopped getting sick."

Like Christianson, a growing wave of parents are turning to elderberry syrup, believed by many to boost immunity, ease symptoms and keep the virus away during a particularly harsh flu season. Even as some doctors remain skeptical, demand for the syrup is high. Elderberry products are often sold out in stores and online, driving many of its devotees to make their own syrup or buy from a stranger online.

"It's insanely popular," said Vicki Larson, wellness manager at Harvest Moon Co-op in Long Lake, which sells a variety of elderberry products, including a locally made syrup. "It's been a huge challenge to keep it on the shelf."

Larson cites two possible reasons why people are suddenly looking for alternative ways to prevent the flu: a nasty flu season, combined with a vaccine that has mixed efficacy, and reports of the negative side effects of prescription Tamiflu, which can include nausea, vomiting and diarrhea.

But does the syrup live up to its claims? Like many natural health remedies, the reviews are varied.

Elderberries are rich in antioxidants and some studies have indicated that elderberry extract can be beneficial in easing cold and flu symptoms.

"Elderberry is a wonderful herb and there's a growing amount of research that shows it can impact the ability for viruses to attach themselves to the body," said Crystalin Montgomery, a naturopathic specialist at Wellness Minneapolis. "It's also relatively safe, but I don't know clinically that I've seen it be super-successful in treating the flu."

The scientific evidence that it works against the flu is not a slam dunk, according to Dr. David Hilden, an internal medicine physician at Hennepin County Medical Center.

"It might help, it might not. We just don't know," he said. "I would not recommend elderberry as a replacement for a flu vaccine."

Kid-tested, mom-approved

Chelsea Markwald began giving elderberry syrup to her two children about four years ago. Now she grows her own elderberry bushes in her backyard in Ham Lake.

"Anytime we are around sick people or have a known exposure to the flu or any other sickness, we take a spoonful of elderberry syrup," the 32-year-old said. "Plus, it's yummy."

Elderberries are about the size of blueberries and are easily grown throughout North America. When boiled down to a syrup and combined with spices and honey, the mixture tastes like a spiced fruit juice.

While most doctors don't recommend elderberry as a replacement for the flu vaccine or other flu-fighting remedies, the elderberry has some merit for its anti-viral and immune-boosting properties.

"There is some evidence-based data for the use of elderberry for influenza," said Courtney Baechler, vice president of the Penny George Institute for Health and Healing in Minneapolis. "It can reduce flu-like symptoms as well as the duration of an influenza infection if started within 48 hours of the onset of symptoms."

Still, some doctors aren't convinced.

"There is 100 percent not enough research on this to say that it's safe," said Dr. Frank Rhame, an infectious diseases expert for Allina Health. "People say, 'It can't hurt' and that is absolutely wrong. You don't know what the downside is until a lot of careful study has been done."

Firsthand experience is enough to convince Diedre Zelin to continue giving her six children elderberry syrup. In lieu of the flu vaccine, Zelin and her family take one to two teaspoons of elderberry syrup daily, and more when sick.

While the syrup hasn't completely kept her family from getting sick this winter, Zelin believes they have recovered faster than if they had not been taking the syrup.

"When you see the benefits firsthand, it makes you want to keep buying it," she said. "We've all been a lot healthier this winter."

Is homemade healthy?

While the flu season has peaked in Minnesota, there is still a lot of influenza going around. That is driving demand for elderberry products, which are difficult to come by.

To get their hands on the elusive syrup, many elderberry fans are making their own syrup or buying it through unconventional channels like Facebook.

Christianson said she sold 500 bottles of syrup last year through the social media platform. This winter? She has already sold 3,000 bottles.

"It sounds crazy and it is," Christianson said. "I'm making a 10-gallon batch every day and I can't keep up with the demand."

To grow her business, Christianson is now working with a scientist so that she can make her syrup in a licensed kitchen and sell it in stores.

For now, she has a cottage food producer license from the Minnesota Department of Agriculture. Aside from posting in Facebook groups, she doesn't advertise and doesn't have a website. Her customers hear about her through word-of-mouth.

Because products made at home by sellers are not regulated or inspected, health experts say there is a risk for toxicity or cross-contamination, and there's no guarantee you're getting what you pay for.

Elderberry leaves and stems produce cyanide, and if the berries are not cooked, they are poisonous, too. The Centers for Disease Control has one known case of poisoning from elderberry juice, but those berries were not boiled before consumption.

"Presuming that because something is natural means it's harmless is irrational," Dr. Rhame said. "It's a hell of a lot more complicated than that."