Birth control pills are highly effective, but there is a lot of medicine that affects the pill and its effectiveness as contraception. Taking two or more medicines can cause an interaction between them, so women should be aware of the potential that hormonal contraception could stop working.
It’s always advisable to speak to your doctor or pharmacist before taking any other medicines alongside the pill, including herbal and complementary medicines that are available without a prescription. Your GP will be able to provide you with more specific advice and guidance.
Why is the contraceptive pill affected by other medicines?
Some medications can cause oestrogen and progesterone levels to drop if you’re taking the pill as well. Certain levels of these hormones are required to prevent ovulation, so lowering these may mean you are longer protected from pregnancy.
The main culprits for causing oestrogen and progesterone levels to drop are enzyme-inducers. They speed up the processing of some contraceptive hormones, lowering levels of these hormones faster than normal, making them less effective. A wide range of medications contain enzyme-inducers, conditions that are often treated with enzyme inductors include epilepsy, depression and HIV, they are also often in treatments for sleep control, weight management, fungal control and even some natural remedies.
Both combined and mini pills are impacted by enzyme-inducers. Other hormonal contraceptive methods can also be affected, such as the patch, vaginal ring and implant. These all use synthetic versions of natural hormones which work together to prevent pregnancy by stopping ovulation. This prevents the fertilisation of an egg by sperm.
Contraception methods not affected by enzyme-inducers are the progesterone-only injection, the intrauterine device (IUD) and the intrauterine system (IUS).
What should I do if I need to take 2 or more medicines?
If you’re on a short course of a supplementary medicine, then it’s advisable to use a temporary form of contraception that won’t be affected. These include condoms, the coil or the progesterone-only injection. Your alternative contraception may need to be in effect for anything from 7 to 28 days after finishing the medicine that interacts with the pill.
For long-term treatment, it may be preferable to avoid medicines mentioned above, but it’s always advisable to speak to your GP when starting or stopping any course of medication.
If you aren’t sure whether your contraception interacts with any other medicine, speak to your doctor, pharmacist, or call the NHS assessment service by dialling 111.
Medicine that affects the pill
Patient information leaflets in medicine packs should alert you to all potential interactions, but here is a list of many common medicines that commonly reduce the effectiveness of the pill.
Which antibiotics affect the pill?
Most commonly used antibiotics such as amoxicillin, trimethoprim and erythromycin do not affect the pill unless they cause significant diarrhoea or vomiting. The exception is rifampin (used to treat meningitis and tuberculosis) which can lower the effectiveness of the contraceptive pill. Brand names of the medicine include Rifadin, Rimactane, Rifater, Rifamate and IsonaRif.
Morning after pill
The ellaOne version contains a medicine called ulipristal that can temporarily reduce the effectiveness of contraceptive pills, so it’s wise to use condoms until your next period. The emergency contraception Levonorgestrel (sometimes branded as Levonelle) should not affect the pill.
Anti-epileptic drugs (AEDs): The risk of pregnancy is higher in women taking the pill who use anticonvulsants to control seizures, epilepsy, migraines and nerve-related pain. These include phenytoin, oxcarbazepine, carbamazepine, rufinamide, topiramate, phenobarbital, and primidone. Popular branded forms of these medications include Tegretol, Topamax, Dilantin, Trileptal, Mysoline, Luminal, and Solfoton.
Anxiety and sleep control medicines: Some AEDs, called barbiturates, are used to control anxiety and sleep disorders, and have been shown to reduce the effectiveness of the pill. These include phentobarbitone, primidone, butabarbital, butalbital, and seco-barbita which have the branded names Luminal, Mysoline and Solfoton.
HIV medication (antiretrovirals): Some medicines used to treat HIV and the hepatitis C virus can reduce the effectiveness of hormonal contraception. The most common of these include ritonavir, nelfinavir, nevirapin, norvir, darunavir, lopinavir, fosamprenavir, tripanavir or velfinavir, branded names include Lexiva and Aptius.
Anti-fungal medication: Medicine such as griseofulvin used to treat infections like thrush, ringworm and athlete’s foot, can have an impact.
Weight loss medication
The anti-obesity medicine Orlistat (Xenical), also available over the counter as Alli, may theoretically affect absorption of the pill by inducing diarrhoea and therefore reducing its effectiveness. Additional precautions should be used.l
Natural supplements and the pill
Certain natural supplements don’t mix well with birth control, the main one being St. John’s Wort, which is used for its anti-inflammatory and antidepressant properties. Also known as hypericum, it’s a yellow flower used in herbal remedies, but research has shown that it interacts with oral contraceptives and is likely to increase the risk of unintended pregnancy.
A 2005 study, published in the journal ‘Contraception’, found that women taking St. John’s Wort daily had 15% less contraceptive hormones in their bloodstream, increased breakthrough bleeding and increased possibility of ovulation. Pharmacists were told to warn women of the potential interactions and offer non-hormonal replacements.
Research has also highlighted concerns that other herbal remedies such as Vitex, Dong Quai, Red Clover and Black Cohosh could also compromise oral contraceptives.
Visit our Online Contraception Clinic for a list of popular combined contraceptives