Herbal products and prescription drugs aren’t always a good mix.

That’s the conclusion of a new study that sheds more light on which herbal remedies should send up a red flag for people on certain drugs.

In addition to dangerous side effects, herbal products can impact the efficacy of the drugs, researchers said.

Their findings were published in The British Journal of Clinical Pharmacology.

The analysis evaluated 49 case reports and two observational studies.

In them, researchers noted 15 cases of adverse drug reactions. Most of the patients had cancer. They were taking alkylating agents and/or kinase inhibitors, such as busulfan, temozolomide, and gefitinib.

Others had a cardiovascular disease and were on warfarin or had undergone a kidney transplant and were taking cyclosporine.

Some of the most problematic substances are herbal products that are widely known, such as St. John’s wort and Ginkgo biloba.

According to the report, herbals such as sage, flax seed, cranberry, goji berry, chamomile, and green tea can interact with some cardiovascular drugs and cause bleeding.

Herbal drugs such as QR and Mentat, used to treat arthritis, and celery root, used to treat menopause, worsened depression in some cases.

The researchers also noted a link between a patient taking phenytoin or valproic acid who had a seizure.

Of the case reports evaluated, researchers said that herb-drug interactions were probable for 51 percent of incidents.

In 37 percent, there were possible herb-drug interactions that were highly probable. In 4 percent of cases, an interaction was doubtful.

Don’t keep doctors in the dark

Overall, the authors warn that people taking herbal supplements such as Ginkgo biloba, Panax ginseng, St. John’s wort, and green tea make sure they check with their doctors to ensure the supplements aren’t known to cause interactions with prescribed medications.

According to the researchers, the failure to disclose the use of herbals is what causes such adverse events.

“Patients’ deliberate refusal to disclose their use of herbal medicines to clinicians have led to underreporting of clinically relevant herb-drug interactions,” the authors wrote.

A survey out last year in The Journal of Alternative and Complementary Medicine (JACM) compared the use of herbal remedies among 171 Hispanic women and non-Hispanic white women.

Of them, 89 percent of Hispanics used the remedies compared to 81 percent of non-Hispanics.

According to the study, fewer than one in six Hispanic women — and one-third of white women — talked about using the products with their doctors.

“Checking herbs and drugs for interactions is very important, and you should be speaking with your doctor and pharmacist about potential interactions,” said Amy Gorin, MS, RDN, a nutritionist in the New York City area.

To find out if your herbal supplements could be detrimental to your health, check out the Natural Medicines Comprehensive Database or the Drug Interactions Checker, Gorin advised.

CVS pharmacies also have a drug interactions checker online that may be helpful, according to Tammy Lakatos Shames and Lyssie Lakatos, nutritionists from New York City.

When herbals hurt

Here are a few herbal products known to cause adverse reactions with certain prescription drugs:

  • Fenugreek. People with diabetes should watch out for this herb, Gorin noted. “It may lower blood sugar levels too much and may interact with some diabetes medications,” she said. “And if you’re taking an anticoagulant (which helps to delay blood clotting) such as warfarin, this could be a dangerous combination because fenugreek may also slow blood clotting.”
  • Melatonin. Don’t mix this with sedatives such as benzodiazepines, narcotics, and some antidepressants. Melatonin makes you tired while sedative drugs also make you sleepy. “You should also avoid taking melatonin if you take anticoagulant medications like Coumadin, as melatonin may slow blood clotting so you may increase your chance of bleeding and bruising,” Lakatos and Lakatos Shames said.
  • St. John’s wort. Don’t mix it with antidepressants, as it could cause the serotonin in your body to rise too much, which can lead to seizures and muscle rigidity. Don’t take it if you’re on birth control or blood thinners either, researchers said.
  • Gingko biloba. Watch out if you take this with fish oil supplements, ibuprofen, naproxen, or aspirin. Though the medications are sold over the counter, they are all blood thinners and can increase the risk of bleeding. Ginkgo biloba slows blood clotting and may also cause bleeding.
  • Echinacea. Be cautious if you take this while on prednisone. Echinacea stimulates the immune system, while the steroid prednisone decreases the immune system, so they interfere with each other, Lakatos and Lakatos Shames said.
  • Schisandra. According to the National Center for Complementary and Integrative Health, if you take this herb and a drug, the amount of the drug in your body may increase, and the drug’s effects can become too strong.

“We think many people are completely unaware that some herbs interact with drugs because they view herbs as natural and without possible side effects,” Lakatos and Lakatos Shames said. “Most people don’t realize that medications are made from herbs, and there can be dangerous consequences from mixing them with other drugs.”