Natural Health Products: A potential or hidden culprit of drug interactions

By Christy Mak, Angela Chen, and Certina Ho

More than 70 per cent of Canadians have used natural health products (NHPs) such as herbs, vitamins, minerals, and homeopathic medicines, which are commonly available in community pharmacies and are regulated by Health Canada. According to a survey conducted by researchers in British Columbia between July 2016 and June 2017, consumers typically use NHPs for minor ailments and chronic health conditions. While NHPs are available without a prescription, this does not mean that they are risk-free and absolutely safe for all. In fact, 12 per cent of Canadians report that they have experienced unwanted side effects from the use of NHPs. Many consumers do not ask their healthcare providers about adverse consequences that NHPs may cause. Therefore, clear communication between healthcare providers and consumers is important to identify potential drug therapy problems and prevent harmful drug-NHP interactions. As the go-to healthcare professionals for medication therapy management, pharmacists should educate patients, as well as other healthcare providers, the proper use and selection of NHPs, advise on potential sides effects and interactions pertaining to NHPs, and be vigilant in reporting suspected adverse events involving NHPs to Health Canada.

NHPs may interact with prescription drugs or with another NHP. For example, they may change the way prescription medications are metabolized and affect how they are absorbed, which may then increase or decrease the drug concentrations in the body. Some NHPs are more commonly used than others. St. John’s Wort is a herbal supplement commonly used for treating depressive disorders. Side effects of St. John’s Wort are generally mild, including upset stomach, dry mouth, headache, fatigue and dizziness. However, St. John’s Wort can affect prescription drug metabolism by inducing liver enzymes in the body, which may cause significant drug-NHP interactions with certain classes of antidepressants. Other common drug interactions with St. John’s Wort include oral contraceptives, certain oral chemotherapy and medications for HIV/AIDS. St. John’s Wort may reduce the efficacy of these medications and lead to treatment failure.


Vitamins and minerals are the most often purchased NHPs by Canadians. Vitamin D and calcium are commonly recommended for adults for osteoporosis prevention; and folic acid is recommended for women of reproductive age to prevent neural tube defects in fetuses. Multivitamins and minerals can also be used to treat deficiencies, poor nutrition, and digestive disorders. Similar to St. John’s Wort, they may cause mild side effects, such as upset stomach and headache. However, some vitamins and minerals are associated with potentially serious interactions with prescription medications. Minerals such as iron, calcium, aluminum, and magnesium may reduce the absorption of some antibiotics, resulting in decreased efficacy and inadequate treatment of the infection; they may also decrease the effects of levothyroxine by interfering with its absorption in the body if taken in close proximity.

To avoid potential drug-NHP interactions, healthcare providers should be comprehensive in information gathering and ask patients about their NHP use. Health professionals should also be aware of the risks and benefits of NHPs and be ready to discuss these considerations with patients. If in doubt, check with the pharmacist. Below are some key points regarding the proper use and selection of NHPs that healthcare providers and patients should be aware of:

  • Look for licensed NHPs (i.e. the eight-digit Natural Product Number (NPN) or Homeopathic Medicine Number (DIN-HM) on the product label) that have been reviewed by Health Canada and deemed to be safe and effective under their recommended conditions of use.
  • Consult a pharmacist before buying and consuming any NHPs.
  • Inform healthcare providers in the patient’s circle of care regarding the use of other medications, over-the-counter drugs, and NHPs. Although NHPs do not require a prescription, they could cause side effects and interact with patient’s other medications.
  • Manage/monitor medication therapy and the use of NHPs to avoid interactions.
  • Report adverse reactions associated with NHPs to Health Canada at https://www.canada.ca/en/health-canada/services/drugs-health-products/medeffect-canada/adverse-reaction-reporting.html.

If Canadians are educated on the above considerations for the use of NHPs, they can safely experience their benefits while avoiding adverse reactions and interactions.

Christy Mak and Angela Chen are PharmD Students at the Leslie Dan Faculty of Pharmacy, University of Toronto; and Certina Ho is an Assistant Professor at the Department of Psychiatry and Leslie Dan Faculty of Pharmacy, University of Toronto.