A list of popular combined contraceptive pills can be found on our website
There are several drugs which interact with the pill and limit its effectiveness. We explain the science behind these drug interactions and signpost a few drugs to be aware of if you’re on the pill.
How do drugs interact with the pill?
The combined contraceptive pill contains two synthetic versions of hormones found in the human body: oestrogen and progesterone. These hormones work together to prevent pregnancy by stopping ovulation and therefore preventing the fertilisation of an egg by sperm.
The main drug interactions that threaten the effectiveness of the pill relate to how these hormones are broken down and absorbed by the body. The main offenders are ‘enzyme inducers’ – these drugs increase the metabolism of these hormones and reduce their concentration in the blood.
Enzyme-inducing drugs put women using combined contraception at risk of pregnancy and breakthrough bleeding.
What drugs limit the pill’s effectiveness?
The following is by no means an exhaustive list, but covers many common drugs that limit the effectiveness of the pill.
Some antibiotics and the pill
Most commonly used antibiotics such as amoxicillin, trimethoprim and erythromycin do not affect the pill unless they cause significant diarrhoea or vomiting.
However, it is widely acknowledged that an antibiotic called rifampin – used to treat meningitis and tuberculosis – can lower the effectiveness of the contraceptive pill by reducing the levels of sex hormones in the body. Brand names of the drug rifampin include Rifadin, Rimactane, Rifater, Rifamate and IsonaRif.
Morning after pill
The morning after pill ellaOne contains the drug ulipristal which can temporarily reduce the effectiveness of contraceptive pills. You should continue to take your contraceptive pill as normal, but also use condoms until your next period.
However, a different form of emergency contraception exists which shouldn’t impact the effectiveness of the pill – Levonorgestrel (generic Levonelle).
Anti-epileptic drugs (AEDs)
The risk of pregnancy is up to 25 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 and phenobarbital, and primidone.
Popular branded forms of these drugs include:
Tegretol , Topamax, Dilantin, Trileptal, Mysoline, Luminal, and Solfoton.
Anxiety and sleep control drugs
Some AEDs called barbiturates are used to control anxiety and sleep disorders, and have been proven to reduce the effectiveness of the pill. These include phentobarbitone, Primidone, Butabarbital, Butalbital, and Seco-barbital.
Popular branded forms of these barbiturates include:
Modafinil, a drug commonly prescribed to treat narcolepsy and sleeping disorders, is increasingly being sold on the black market as a concentration-enhancing ‘smart drug’.
It is popular among students although many do not realise that it can reduce the effectiveness of the pill and its results may last for up to two months.
Branded forms of Modafinil include:
Provigil, Modavigil, and Alertec.
HIV medication (antiretrovirals)
Some drugs used to treat HIV can reduce the concentration of oestrogen and progesterone in the blood. The most common of these contain ritonavir, nelfinavir, nevirapin, norvir, darunavir, lopinavir, fosamprenavir, tripanavir or velfinavir. Some of these drugs are also used in the treatment of the hepatitis C virus.
Popular branded drugs containing protease inhibitors include:
Some anti-fungal drugs such as Griseoulvin used to treat infections like thrush, ringworm and athlete’s foot, can limit the effectiveness of the pill.
Weight loss medication
The anti-obesity drug 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 precaution should be used.
Natural supplements and the pill
Certain natural supplements can also limit the effectiveness of combined contraceptive pills, including St. John’s Wort, which is used for its anti-inflammatory and antidepressant properties.
Some studies have also suggested that the chemicals contained in grapefruit can boost rate at which the body clears sex hormones contained in the pill.
What should I do?
There are several options available to women about to start any of the above courses of medication.
Women can continue to take the combined pill as normal, but use a temporary additional form of contraception which isn’t affected by the above drugs. These include:
- Progesterone-only injection
- The coil (IUD)
These ‘back-up’ forms of contraception may need to be used for 7 to 28 days after completing the course of medication which interacts with the pill. For women who need long-term treatment involving drugs that interact with the pill, it may be preferable to switch to one of the above entirely.
Crucially, you should not stop or start any medicines you are taking without speaking to your doctor first. Always let them know the contraceptive pill you’re taking if you’re starting another medicine.
Visit our Online Contraception Clinic for a list of popular combined contraceptives: