Women who take the morning-after pill are being warned that those taking common medications may need a double dose after research found 400 unwanted pregnancies.
Medicines regulators have issued a public warning, explaining that some medicines and herbal remedies can interfere with the emergency contraception.
The Medicines and Healthcare products Regulatory Agency (MHRA) issued new guidance over the pill after research found hundreds of unwanted pregnancies among those taking it.
The MHRA said that women taking medicines for epilepsy, herbal remedies containing St John’s Wort, and drugs for fungal infections might need a double dose of the emergency contraceptive for it to be effective.
Drugs for tuberculosis and HIV were also found to interfere with the effectiveness of the pills.
The watchdog said that the non-hormonal coil was the most suitable form of emergency contraception for many of those taking the daily medications.
But if this was not possible, a double dose of the morning after pill was recommended, experts said.
Dr Sarah Branch, deputy director of MHRA's vigilance and risk management of medicines division, said: "This is important new advice for women who want to use the emergency contraceptive pill.
"It will help to protect women who are taking certain medicines against unwanted pregnancies.
"Our new patient information sheet provides information on what types of medicines could interfere with how the emergency contraceptive works.
"It tells women what steps they need to take to ensure they receive the correct dose.
"The earlier that emergency contraception is taken after unprotected intercourse, the better it works."
The MHRA said it had data on 400 unwanted pregnancies in women taking levonorgestrel since it was first licensed in the 1970s. A spokesman said it was not clear how many of these pregnancies were due to interactions with other drugs, but said research had found a link.
Levonorgestrel - marketed under brands including Levonelle and Plan B - has to be taken within 72 hours (three days) of unprotected sex.
The drugs that can interfere with it include carbamazepine, barbiturates, primidone, phenytoin for epilepsy, rifampicin and rifabutin for TB, ritonavir and efavirenz for HIV, drugs for fungal infections, including griseofulvin, and any herbal remedies containing St John’s Wort, which is used to treat depression.
The watchdog said it first became aware of the issue in 2014, and had issued the new advice after a review by the European Medicines Agency, which completed in May of this year.
The new information sheet will be available through GP surgeries and pharmacies.
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