St John's wort may make the morning after pill less effective. [Photo: Bigstock]

Q&A: Does St John’s wort interfere with the Pill?

21 September, 2016

Q — I have recently seen some news stories suggesting that St John’s wort can stop the Pill working. Is this really true?


A — A recent news story noted that women taking an emergency contraceptive pill containing levonorgestrel are being advised to check the ingredients of other medicines they are taking because they could stop the pill working.

Levonorgestrel is synthetic hormone and is used in type of emergency contraception which can be taken within three days of unprotected sex to prevent unwanted pregnancies.

However, certain ingredients in other medicines significantly reduce the effectiveness of the levonorgestrel pill.

The Medicines and Healthcare products Regulatory Agency (MHRA) advisory implicated several conventional medicines including those for:

  • epilepsy (e.g. medicines called barbiturates, primidone, phenytoin, or carbamazepine)
  • tuberculosis (e.g. rifampicin, rifabutin)
  • HIV (e.g. ritonavir, efavirenz)
  • fungal infections (e.g. griseofulvin)

Herbal remedies containing St John’s Wort (Hypericum perforatum) were also implicated.

Herbal medicines are powerful bioactive substances. It is important to always tell your doctor what pills you are taking – even if they are herbal ones. The interaction between St John’s wort and contraceptive pills, including the morning after pill, is well-known and any healthcare professional you seek advice from (conventional or complementary) will need this information to help you make the right choices.

In the case of the morning after pill the MHRA advises that women taking St John’s wort should inform their pharmacists because they may need a higher dose of levonorgestrel for it to be effective.

In response to a MHRA advisory the chair of the BHMA, Dr William House, noted:

“We have known for many years that St John’s Wort can prevent hormonal contraceptives and the “morning after pill” from working effectively. Over-the-counter St John’s Wort products in the UK are regulated by the Medicines and Healthcare Products Regulatory Agency (MHRA) and contain an MHRA approved in-pack leaflet which gives details of safe and appropriate use of the product.

“The leaflet gives details of potential drug interactions, side-effects, dosage and how to take the product, as well as advice on when to seek help from a medical practitioner. These regulated products can be identified by a THR logo on the carton and leaflet, and it is important to always read the leaflet before taking in order to ensure that it is safe and appropriate to use.”

The MHRA publishes an online factsheet for helathcare professionals which details St John’s wort interactions with other medications.