All natural. Not all safe.

But there’s also no guarantee that what you’re buying will cure your cold, silence your arthritis, or calm your nerves. And it might even spark serious side effects - especially if an otherwise innocuous herb is taken with a powerful prescription pill, producing a toxic brew.

This month, an over-the-counter cold treatment called Zicam landed in the cross hairs of the US Food and Drug Administration after the agency received more than 100 reports of users losing their ability to smell - sometimes briefly, sometimes permanently. The FDA’s advice: Stop using Zicam sprays and gels. Immediately. It was the latest cautionary tale about products that, in the main, fly under the radar of regulatory agencies whose main job is approving and monitoring prescription medications.

Americans spend billions of dollars annually on pills, herbs, and other medical products known variously as alternative, complementary, natural, and homeopathic. And the users - including some traditionally trained physicians - swear by them, even if federal drug regulators don’t. Manufacturers argue that many of the products - especially herbs - have survived the crucible of time, used through the ages without complaint. Consumer advocates counter that longevity is no substitute for gold-standard scientific evidence.

This much is clear: More than five years after the FDA banished an herbal weight-loss compound called ephedra that was blamed for 155 deaths, the love affair with supplements blazes hotter than ever. Never has it been more important for patients and doctors alike to understand the potential promise and peril carried in the amber bottles of hope stocked on bulging shelves.

“You’ll find one person who says this is the next best thing since sliced bread, and somebody else who says this will kill you. But, really, the truth is somewhere between,’’ said Candy Tsourounis, a professor of clinical pharmacy at the University of California-San Francisco. “So how does a consumer get the truth from these two polar opposites? It’s going to the right provider, asking the right questions, being informed.’’

When Dr. Benjamin Kruskal encounters patients for the first time in his Somerville office, he lets them know he harbors no hostility toward alternative medicine.

“The biggest thing I communicate to people is caution and looking hard for evidence about safety - above all else, safety first,’’ said Kruskal, who practices at Harvard Vanguard Medical Associates. “And I do encourage people to tell me what they’re using. A lot of people say, ‘I’m taking vitamins, that doesn’t count as a medicine.’ But if they’re taking 3 grams of vitamin C, it sure does count as medicine.’’

It’s important for physicians to know whether patients are taking herbs or other products that can react powerfully with prescription medications, sometimes amplifying the effect of those drugs, sometimes muting it.

For instance, the herb St. John’s wort, which has been shown in some scientific studies to alleviate depression, can impair the liver’s ability to process certain drugs. That’s why HIV patients taking medication called protease inhibitors are urged not to use the herb. There’s also evidence that St. John’s wort can make birth control pills less effective.

Patients who take the blood- thinning drug Coumadin - sometimes prescribed after a heart attack or stroke - are told to be especially forthright with their doctors when discussing supplements they take, because some over-the-counter products have been shown to alter the medication’s function.

Herbs and homeopathic products, the class that includes Zicam, do not have to undergo elaborate, expensive scientific studies proving they work before they can be marketed - unlike prescription medications. Nor do they require the FDA’s approval, although manufacturers must promptly alert regulators if they receive reports of serious side effects linked to their products.

(Of course, as scandals with some prescription drugs have so clearly demonstrated - think Vioxx - the FDA’s approval process is hardly foolproof.)

Companies that make supplements and herbs, much like food processors, are required to hew to good manufacturing practices. But those rules are no assurance that the ginkgo you buy in one shop is the same strength as the ginkgo peddled down the street.

“One thing we can say for sure is if consumers are contemplating using a medication, they should familiarize themselves quite well with what’s on the label,’’ said Dr. Charles Lee, an FDA medical officer.

Evidence that supplements truly help is spotty - and, sometimes, bewilderingly mixed.

Consider the widely used herb echinacea, heralded for its purported ability to dampen, or even prevent, respiratory infections. Scientists at the National Center for Complementary and Alternative Medicine - it has a trove of information at www.nccam.nih.gov - declare that when it comes to preventing colds or the flu, echinacea is a bust.

The findings are more equivocal regarding the herb’s ability to treat a cold once it has already settled in. While two studies funded by the federal agency concluded there was no evidence echinacea can ease a cold, other studies have shown it may indeed help.

“The issue is this,’’ said Dr. Sidney Wolfe, acting president of Public Citizen, a leading consumer advocacy group whose research has seared prescription products as well as alternative supplements: “Anything that is supposed to affect the human body should have scientific evidence that it affects the human body in a good way and that it is safe.’’

Michael McGuffin, president of the American Herbal Products Association, a trade alliance, has an answer for Wolfe: Pay attention to history.

“If I’m marketing an herbal ingredient that’s used for a purpose that’s known and consistent for hundreds of years . . . do I need’’ a gold-standard scientific trial, McGuffin asked. “I don’t think so. I have respect for these uses that have been recorded for centuries. I just don’t see that there’s a need for a new scientific model to confirm what we already know from experience.’’

Sometimes, deciding whether to use a supplement for a relatively minor ailment comes down to an exquisite balancing of benefit and risk.

Such was the case with Dr. Rick McKinney, who melds mainstream health practices with the less conventional approaches favored by some of his patients. A few years back, the fog of a cold descended upon him.

He could have just soldiered on, enduring the drip, drip, drip. But a buddy had touted Zicam. “By God, it worked - it was a huge reduction in the nuisance symptoms of the common cold,’’ said McKinney, a UC-San Francisco physician who acknowledges his was a scientifically irrelevant “study of one.’’

It proved to be the only time he used the spray. McKinney decided to dig a little and discovered that some Zicam users reported losing their sense of smell after squirting the zinc-laced compound up their nostrils.

“I wasn’t willing to take the chance,’’ McKinney said. “If this was a chance to cure me of cancer, sign me up. But this was a chance I was taking to avoid nuisance-level symptoms and not something that was going to make me live any longer - even if it worked well.’’

Stephen Smith can be reached at stsmith@globe.com.