The Pill and Other Medicines: What Actually Matters
Why ordinary antibiotics are not a problem, which medicines genuinely reduce contraceptive effectiveness, and the interaction that runs the other way.
Part of the Complete Contraception Guide.
Key fact: Ordinary antibiotics do not reduce the pill's effectiveness. The guidance changed years ago and the myth outlived it. What does matter is a much smaller group of enzyme-inducing medicines — and vomiting or diarrhoea from the illness itself.
The antibiotic myth
For decades, anyone prescribed antibiotics while on the pill was told to use condoms for the duration and a week afterwards. That advice was withdrawn from UK guidance, but it survives in waiting rooms, in leaflets, and in what people tell each other.
The current position is unambiguous: no additional contraceptive precautions are required when taking an antibiotic, unless one of two things is true.
- The antibiotic is itself an enzyme inducer — in practice this means rifampicin or rifabutin, which are used for tuberculosis and a small number of other indications, not for a chest or urine infection
- The antibiotic, or the illness being treated, causes vomiting or diarrhoea — which affects absorption, and is covered in vomiting, diarrhoea and the pill
So amoxicillin, doxycycline, trimethoprim, nitrofurantoin, clarithromycin, flucloxacillin, metronidazole and the rest do not affect your pill. If you are being treated for an infection and feeling well enough to keep the tablets down, your contraception is working as normal.
What does reduce effectiveness
Enzyme-inducing drugs speed up the liver's breakdown of oestrogen and progestogen, lowering hormone levels enough to matter. Breakthrough bleeding is often the first sign.
| Group | Commonly involved medicines |
|---|---|
| Antibiotics that are enzyme inducers | Rifampicin, rifabutin — potent inducers |
| Anti-epileptics | Carbamazepine, phenytoin, phenobarbital, primidone, topiramate |
| HIV medicines | Efavirenz and some boosted regimens |
| Herbal | St John's wort |
This is not an exhaustive list, and it changes. Guidance now directs prescribers to check the individual medicine in the BNF rather than working from a fixed list, so the safest habit is to ask specifically — "does this interact with my contraception?" — every time something new is started. Not every anti-epileptic is an inducer; sodium valproate and levetiracetam, for example, are not.
Which methods are affected, and which are not
| Method | Affected by enzyme inducers? |
|---|---|
| Combined pill, patch, ring | Yes — effectiveness may be reduced |
| Progestogen-only (mini) pill | Yes |
| Implant | Yes — a point often missed, since it is otherwise the most forgiving method |
| Oral emergency contraception | Yes |
| Injection | No expected effect |
| Hormonal coil (LNG-IUS) | No expected effect |
| Copper coil | No effect |
The implant is the surprise on that list. People reasonably assume that a method requiring nothing of them is immune to interactions, and it is not. The coils and the injection are the options that genuinely sidestep the problem.
Short course versus long-term treatment
- A short course of an enzyme inducer: use condoms during the course and for a period afterwards, because enzyme induction persists after the last dose — about four weeks is the usual advice. Your prescriber will confirm the interval for the specific drug.
- Long-term treatment: working around the interaction indefinitely with condoms is not a sensible plan. The usual answer is to change to a method that is unaffected — a coil or the injection — rather than to adjust the pill.
- Do not simply take extra pills. Dose adjustments to work around induction exist in specialist practice but are not something to improvise.
Medicines the pill itself affects
Interactions run both ways, and the most important example is lamotrigine, used for epilepsy and bipolar disorder. The combined pill lowers lamotrigine levels, which can mean loss of seizure control — and levels rise again during the hormone-free break, so a cyclical pill produces a swing every month.
If you take lamotrigine, do not start or stop a combined pill without speaking to the clinician who manages it. The lamotrigine dose may need to be increased, potentially as much as doubled, with blood level monitoring — and stopping the pill without lowering the dose again can push levels too high. Progestogen-only methods do not interact in this way.
The combined pill also interacts with some immunosuppressants and thyroid replacement in ways that need monitoring rather than avoidance. The point is not to be alarmed but to make sure both prescribers know about each other.
Herbal remedies and over-the-counter products
- St John's wort is a genuine enzyme inducer and reduces contraceptive effectiveness. It is sold without prescription for low mood, which is exactly why it gets missed.
- Most other supplements — vitamins, iron, evening primrose, cranberry — have no effect on the pill.
- Activated charcoal and some anti-diarrhoeal products can interfere with absorption if taken close together; separate them by a few hours.
- Grapefruit juice can raise oestrogen levels slightly. It is not a contraceptive risk and needs no action.
Tell whoever prescribes your contraception about anything you buy yourself, including herbal products. "Natural" and "no interaction" are unrelated ideas.
Interactions and emergency contraception
Enzyme inducers reduce oral emergency contraception too, which changes the choice:
- A copper coil is preferred, because it is unaffected
- If a tablet is used, levonorgestrel is given as a double dose (3mg)
- Ulipristal is not doubled — doubling it is not recommended
Emergency contraception compared covers the rest of the decision, including the five-day wait before restarting a pill after ulipristal.
What to do if you are prescribed something new
1
Say what contraception you use, by name
"I'm on the pill" is not enough — combined, mini pill, implant and coil behave differently.
2
Ask directly whether it interacts
A pharmacist can answer this at the counter in under a minute.
3
If it does, ask how long the effect lasts after the course
Induction outlasts the last dose, so the condom period extends beyond the end of the treatment.
4
Treat new breakthrough bleeding as a signal
Unexpected spotting after starting a new medicine is worth mentioning rather than waiting out — see bleeding between periods.
Frequently Asked Questions
Do antibiotics stop the pill from working?
No. Standard antibiotics do not reduce the effectiveness of hormonal contraception and no extra precautions are needed. The exceptions are rifampicin and rifabutin, which are enzyme inducers, and any situation where the antibiotic or the illness causes vomiting or severe diarrhoea.
Which medicines make the pill less effective?
Enzyme-inducing drugs, which speed up the breakdown of contraceptive hormones. Commonly involved are rifampicin and rifabutin, carbamazepine, phenytoin, phenobarbital, primidone and topiramate, efavirenz and some HIV regimens, and St John's wort. Lists change, so ask about any new medicine specifically.
Is the implant affected by other medicines?
Yes. Enzyme-inducing drugs can reduce the effectiveness of the implant as well as the pill, which surprises people. The injection, the hormonal coil and the copper coil are not affected, so they are the usual alternatives for someone on long-term enzyme-inducing treatment.
Does St John's wort affect the pill?
Yes. St John's wort is an enzyme inducer and reduces contraceptive effectiveness. Because it is bought over the counter for low mood rather than prescribed, it is often not mentioned, so tell whoever prescribes your contraception if you take it.
I take lamotrigine. Can I take the pill?
Not without discussing it with the clinician who manages your lamotrigine. The combined pill lowers lamotrigine levels, which can affect seizure control, and levels rise again during the pill-free break. The dose may need adjusting with monitoring. Progestogen-only methods do not interact this way.
References
- Faculty of Sexual & Reproductive Healthcare. Combined Hormonal Contraception (amended October 2023). fsrh.org
- Faculty of Sexual & Reproductive Healthcare. Progestogen-only Pills (August 2022, amended 2026). fsrh.org
- Faculty of Sexual & Reproductive Healthcare. Emergency Contraception (March 2017, amended 2026). fsrh.org
- Faculty of Sexual & Reproductive Healthcare. Drug Interactions with Hormonal Contraception (May 2022). fsrh.org
- Faculty of Sexual & Reproductive Healthcare. Problematic Bleeding with Hormonal Contraception. fsrh.org
- Faculty of Sexual & Reproductive Healthcare. UK Medical Eligibility Criteria for Contraceptive Use (UKMEC 2025). fsrh.org
- NHS. Contraception. nhs.uk
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Contraceptive choice depends on your medical history, and no method described here protects against sexually transmitted infections. Always read the leaflet supplied with your own pill and consult a qualified healthcare professional. In a medical emergency, call 999.