Katie DeRosa, a 26-year-old graphic designer in Lawrence, Massachusetts, went to see her allergy doctor in 2014 about a nagging post-nasal drip. The doctor suggested she try the popular over-the-counter antihistamine Zyrtec, which she started taking regularly.

Four years later, DeRosa tried to stop taking the medication. She soon wished she’d never started.

After two days with no Zyrtec in her system, DeRosa “suddenly became extremely itchy all over my body, even in the most random places like the middle of the palm of my hand,” she says. “It was bad enough to make me unable to concentrate on anything. All I could think about was how horribly itchy I was all over. I’ve never felt anything like that before.”

After two hours of the itch from hell, she caved and took a Zyrtec pill. Within an hour, her symptoms had disappeared.

“That’s when I realized that something was wrong,” DeRosa says.

DeRosa’s experience is not unique. Within the whisper network of online message boards, you’ll find hundreds of people who claim to have similar struggles when they tried to quit taking Zyrtec. They describe their withdrawal with words such as “heroin-like” and “total torture” and phrases like “the worst thing I’ve ever endured.” Some report they “scratched spots on [their] hands until they bled” and feeling “itchy for 3 days straight and am in total hell.” They debate whether to quit cold turkey or to taper off the meds.

The evidence is not just anecdotal. A 2016 study from the Netherlands published in the journal Drug Safety — Case Reports described 12 people who experienced severe itching (also called pruritus) after attempting to stop taking cetirizine or levocetirizine — the generic names for Zyrtec and a chemically similar drug sold as Xyzal. The study and subsequent local response led to a label change for the drug in the country.

But no such warnings exist for Zyrtec sold over the counter in the United States. Its drug labels and packaging only cite the possibility of drowsiness and advise checking with a doctor before using if you have liver or kidney problems or are taking sedatives. It’s not that health regulators in the U.S. are unaware of the possible side effect. The U.S. Food and Drug Administration (FDA) says in an email to Medium that the agency is “aware of this concern and is continuing to look at this issue.”

In a statement to Medium, Johnson & Johnson, the makers of Zyrtec, said, “Millions of people around the world rely on Zyrtec to relieve their allergy symptoms. The safety of patients who take our products is our top priority. Reports of pruritus upon withdrawal is categorized as very rare. In compliance with regulatory requirements, we report adverse event data to local market regulatory authorities, and will continue to rigorously monitor and report adverse safety events.”

I interviewed over a dozen people for this story and read the posts of hundreds more. Most stories follow a pattern similar to DeRosa’s. A person is grappling with allergies or hives. They seek out Zyrtec or its generic equivalent, cetirizine, for relief. Maybe a doctor recommended it, or they found it themselves on a pharmacy shelf. They use the pills regularly for months or years. Then, perhaps at the end of a spring hay fever season, they try to stop.

It was much more than my usual allergy symptoms returning. It was a fiery sensation, an itch that felt like it was bubbling up from inside me.

As if on a timer, when 24-48 hours have passed since their last cetirizine dose, the agonizing itching suddenly sets in. Imagine your scalp, the palms of your hands, and the soles of your feet afflicted with an intense pins-and-needles sensation or constant full-body pinpricks that scratching yourself red won’t relieve even though you may try.

I can relate to their struggles. Like many of us, I take over-the-counter medications like Advil and Pepto-Bismol for my headaches and indigestion without even thinking twice about it. When I moved from Florida to New England in 2006, my already allergy-prone immune system balked at the smorgasbord of new regional pollens. So I took a 10 milligram Zyrtec pill daily for five years to control my intense allergies. It worked. But then I tried to quit.

Exactly two days after taking my last pill, the severe itching set in. It was much more than my usual allergy symptoms returning. It was a fiery sensation, an itch that felt like it was bubbling up from inside me. I’d sit at my cubicle at my library job, scratching at my scalp and palms all day, praying no one noticed my itching or my raw skin. I tried to brace through it, thinking it would eventually subside. But inevitably, I’d take a Zyrtec to make the itch go away.

So began a vicious cycle of attempts to quit. My body demanded cetirizine every 48 hours. The pill packaging said nothing about withdrawal symptoms, but the constant chain of relapses made me wonder if I’d have to stay on Zyrtec forever.

It makes me feel crazy to talk about this publicly: I ostensibly got hooked on drugstore medicine. But I always wondered, how real are my and others’ experiences? How could an over-the-counter allergy pill that millions of people take — including children — be so hard to quit?

How real is Zyrtec “addiction”?

Allergies occur when your immune system interprets a foreign substance — anything from dust mites to a peanut — as a threat and produces antibodies to attach to it and attack it. This allergen-antibody duo releases histamines, chemicals made by your immune system that try to keep the body “safe” by expelling the foreign substance. This process causes symptoms familiar to allergy sufferers: runny noses, itchy eyes, sneezing, congestion, and in some severe cases, anaphylaxis.

Antihistamines, like Zyrtec, were first discovered in the 1930s by an Italian chemist named Daniel Bovet. By the 1940s, some of the antihistamine formulas Bovet discovered, such as pyrilamine, began to be distributed to consumers. These drugs tended to be sedating, however, and by the 1980s, scientists had created newer antihistamines that were less likely to cause drowsiness. Unlike the older first-generation antihistamines, second-generation medications — which today include the holy trinity of Claritin, Allegra, and Zyrtec — don’t cross the brain barrier as much.

“Newer, second-generation antihistamines are less likely to make you feel sleepy and generally last longer,” says Dr. Kathleen Dass, an allergist and immunologist based in Oak Park, Michigan.

Today you can buy Zyrtec over the counter, but it was initially sold as a prescription-only drug in the U.S. until 2008.

Zyrtec was first manufactured in the 1980s by a pharmaceutical company headquartered in Brussels called Union Chimique Belge (UCB). UCB eventually licensed a U.S. company called Pfizer to promote the drug domestically, and in 1996, UCB and Pfizer obtained FDA approval for Zyrtec and started selling it as a prescription drug.

Zyrtec quickly turned into a mega business for the UCB/Pfizer camp. The drug was credited with galvanizing a 19-percent year-over-year increase in sales for 2001. In the U.S. alone, Pfizer’s revenues grew by 44.1 percent to over 1 billion dollars that year. At the time, Zyrtec was one of the top-selling prescription drugs in America.

In 2006, Johnson & Johnson acquired Pfizer Consumer Healthcare, and with it Zyrtec, in a $16.6 billion deal. But UCB’s U.S. patent on cetirizine expired in 2007, the same year that the drug was approved for over-the-counter sales. UCB also developed a slightly different drug called levocetirizine, which goes by the brand name Xyzal, which it licensed to Pfizer. That drug received FDA approval only six months before Zyrtec’s patent expired.

With the patent’s expiration, Zyrtec was open to the free market. Not only did generic cetirizine hit the shelves in 2008, but the FDA allowed it to be sold over the counter. The FDA stated in a 2007 Wall Street Journal article that “allergy medicines can be switched to nonprescription status because consumers can diagnose the conditions themselves and they’re safe without supervision.”

Though now available over the counter and in generic form, revenues for brand-name Zyrtec continued to soar. Not much has slowed that trend today: According to data from Statista, Zyrtec was the top OTC allergy medication drug in 2016, generating almost $348 million in revenue that year. Today you can buy Zyrtec in tablet, liquid gelcap, and fast-dissolve melt forms; there’s also a children’s syrup for tiny allergy sufferers, which the label says can be dosed with up to 10 milliliters of cetirizine for severe symptoms.

Peter, 27, who doesn’t want his last name published for privacy reasons, first started taking Zyrtec when he was a 26-year-old student studying in Switzerland. He had developed a chronic itch while studying for exams, and his dermatologist suggested Zyrtec.

After two years of taking the allergy drug daily, Peter decided he wanted to stop, only to realize how dependent he had become on it.

“Unlike the itch I started taking it to cure, the itching had become predictable. It would start exactly 24 hours after taking my last pill of Zyrtec and was far more intense,” he says. “An isolated, sporadic itch had become a full-body maddening itch.” He began to research his symptoms.

Over 900 people had reviewed Zyrtec-brand tablets on the product’s official website at the time of this post, and over 86 percent of them gave the product five out of five stars. “Zyrtec has changed my life,” raves one reviewer. “I take it 365 days of the year and live an allergy free life. Thank God for Zyrtec!” (Zyrtec’s official review guidelines grant the company the right to remove any reviews that mention “any undesirable experience associated with the use of the product.”)

On certain non-Johnson & Johnson websites, reviewers tell a different story. At the time of this post, 64 percent of the Zyrtec reviews with star ratings on Consumer Affairs gave the product one star. “If there is any way for you not to start taking Zyrtec, don’t start!” writes one user. “My doctor put me on it about five years ago for itching. One vacation I forgot to take it for three day [sic] — and experienced terrible, incessant itching. Within 10 minutes of taking a tablet I was fine.”

Sites from Reddit to The People’s Pharmacy are littered with anecdotes from people relieved to find others who were describing the exact same predicament. Of course, it’s not possible to verify every anonymous claim, but the fact that there are so many threads and forums devoted to the problem raises questions.

“I’d lie awake for hours and just count the spots that were itching on me. I didn’t realize at the time, but it was because I didn’t bring my Zyrtec to his house.”

“I thought I was going crazy until I read online that others had the same issues,” writes Wendy of Madison Heights, Virginia on the Consumer Affairs website. “I feel like there is [sic] a million fire ants running all over stinging me for the past 3 weeks,” says Vanessa of Batavia, Ohio. In a closed Facebook group called Kicking the Big Z (Zyrtec or Cetirizine Withdrawal), almost 500 members debate coping strategies. People who have quit successfully give advice on how to wean off.

Kelly, who prefers her last name be withheld for privacy, is a 32-year-old retail assistant manager and service dog trainer in Minnesota who looked to Reddit to help quit Zyrtec. She first experienced the Zyrtec itching when she started staying overnight at her now-husband’s house.

“I would be so itchy at night,” she says. “Excruciating. I’d lie awake for hours and just count the spots that were itching on me. I didn’t realize at the time, but it was because I didn’t bring my Zyrtec to his house.”

Ten years later, she tried to set up an appointment with an allergist after suffering from congestion during the night. “They told me I had to stop taking any allergy medicine for a week before the appointment,” she says. The intense itching soon set in. “And that’s when I started to notice the correlation,” Kelly says.

In 2016, researcher Florence van Hunsel and her colleagues at the Netherlands Pharmacovigilance Centre Lareb — which maintains the country’s reporting system for adverse drug reactions — noticed a number of similar reports coming in with the same symptoms: pruritus upon stopping regular cetirizine use. They decided to analyze the reports in a formal case study, published in 2016.

The patients described pruritus that occurred one to three days after withdrawal of long-term use of the drugs without any other symptoms such as rashes or urticaria (hives). Nine of the people reported previous attempts to stop using cetirizine and levocetirizine that failed because of the “unbearable itch.” The case studies included 11 women and one man, with a median age of 39; four cases were reported by health care professionals, and eight were reported by consumers.

A spokesperson for the FDA said the agency “is aware of this concern and is continuing to look at this issue.”

“We received a number of the reports described in the article, and we noticed that this reaction was not yet described in the product information/patient information leaflet and that the story described in all reports was very similar,” says van Hunsel, who serves as head of signal detection for the center. Van Hunsel and her co-authors concluded that their findings “suggest a causal relation between withdrawal of (levo)cetirizine and the occurrence of unbearable pruritus.”

Many people I spoke with for this story said they’d submitted complaints to the FDA about Zyrtec’s side effects. A search of the public dashboard of the FDA Adverse Effects Reporting System (FAERS) shows more than 14,000 cases reported of Zyrtec tablets since 1996; over 800 of those mention pruritus as the adverse reaction.

When I asked the FDA about these complaints, Sandy Walsh, a spokesperson for the FDA, said the agency is aware of the issue, writing that “the potential safety issue of rebound pruritus with cetirizine, levocetirizine, and hydroxyzine was posted July-September 2017, prompting an evaluation to determine the need for regulatory action.”

As a result of the review, Walsh says the “prescribing information” for prescription levocetirizine was revised to include the information about pruritus after discontinuation of cetirizine. But the prescribing information for a drug is detailed information meant for physicians, not for consumers. It’s not contained on a package or drug label, and a consumer wouldn’t receive that kind of information at the pharmacy.

The changes also don’t include Zyrtec. Regarding the itching symptoms after quitting Zyrtec, Walsh says, the “FDA is aware of this concern and is continuing to look at this issue.”

It can be challenging to have possible Zyrtec withdrawal symptoms taken seriously. When I sought medical advice, I was told my itching was just the return of my normal allergic symptoms. But I’d never experienced bodily itching before starting cetirizine.

The doctors I talked to about Zyrtec withdrawal for this story all spoke with ambiguity. “The mechanism of Zyrtec is to block the histamine (itch) receptors,” says Dr. Amy Shah, an Arizona-based allergist and immunologist, “so it is plausible that by stopping it, you can get itching. But I am not sure if this is a cause of Zyrtec or if it’s because the itch receptor is now unblocked.”

“These are case reports [of cetirizine withdrawal itching], but it’s not really substantiated by large studies,” she adds.

Dass, the Michigan allergist and immunologist, says the phenomenon is “very real, though also less common.” She’s only seen one patient with these symptoms. “I worked with her to slowly taper the medication, and her symptoms resolved after a few days,” she says.

“Zyrtec and Xyzal withdrawal are rare,” she adds. “If people are concerned about this possibility, physicians can always use Claritin or Allegra, which are non-sedating and do not have reports of the withdrawal effect.”

Michael Kachel, the director of pharmacy of Allergychoices, Inc. in Onalaska, Wisconsin, says he’s heard reports of post-cetirizine itching, though he hasn’t personally treated a patient with these symptoms.

“It’s not like the itching people experience after stopping their cetirizine is a lasting effect. It’s uncomfortable, but the body does kind of rebound again,” he says. “By slowly tapering off the drug versus blatantly stopping, it should help minimize the effect.”

“Antihistamines are short-term fixes, and they shouldn’t be taken long term unless under the advice of your physician,” he adds.

“Discontinuation syndrome” is an FDA-acknowledged term for the symptoms that occur in patients who abruptly stop a medication. One question is whether a similar one should be added for medications like Zyrtec.

“Package inserts of medications are required by the FDA to list reported side effects,” Dass says. “I do think the package insert should reflect withdrawal as a rare but possible side effect. I also think that more research should be done to explore why these symptoms are occurring and who is most at risk.”

The online community suggests that if you can withstand the crazy itching for four to six weeks, it will eventually subside. Some describe coping mechanisms like brushing their skin or soaking in oatmeal baths. Doctors, if they agree to help, can recommend a tapered dosage aided by a corticosteroid such as prednisone, which reduces inflammation by lowering the body’s immune system.

Peter says he was able to taper down his dose over six weeks and then managed to stop cold turkey. “Here I am today, itch-free and Zyrtec-free for six months,” he says. “The whole process, to put it bluntly, really really sucked. I’m just glad to be back to ‘normal’ because it was scary to be so dependent on a pill without a definitive answer for what was causing my itch.”

Kelly eventually stopped her Zyrtec habit with the help of another antihistamine. “After some deep googling,” she says, “I found some tips to wean myself off. I started by taking Claritin every other day at first, which made the itching bearable but miserable. Then after a few weeks, I was able to switch completely to Claritin. With Claritin, if I miss a day, I don’t even notice [any side effects] except sniffling.”

DeRosa also managed to swap medications. “I decided to try taking Allegra daily for two weeks, then switched to every other day for a few days, and then stopped cold turkey,” she says. “I didn’t want to become reliant on another allergy medicine.”

The 2016 Dutch study of 12 patients found that one person reported that a gradual reduction in dose helped, and after several weeks, “the itching slowly receded and eventually disappeared.” Another patient used a short course of corticosteroids, and five reported using concomitant medication where they substituted cetirizine with another antihistamine to help taper off.

I personally quit for good by weaning myself off, a method I read about online from other sufferers. I carefully bisected my white pills with a kitchen knife, trying to ignore the jagged edges as they scraped my throat. Once I had stepped my daily dose down to 5 milligrams per day, I switched to fexofenadine (Allegra), and the itching finally, totally disappeared.

Today, I’ve found relief from my allergies with the prescription antihistamine nasal spray azelastine as well as occasionally taking an Allegra gelcap during hay fever season. But I can still feel the ghost of the itching sometimes and sense phantom crawling on my palms and scalp. I’ll never again take Zyrtec.