how effective?

Emergency Contraception.

Emergency contraception generally comes in pill form. The pack contains two [or four or even 40!!] pills of very high dose contraceptives that aim to prevent you from getting pregnant after you’ve had an accident or unprotected sex. In the UK “levonelle” is used. They’re effective up until 72 hours after sex, but are best taken as soon as possible.

Side effects: [rare]

• headache
• tummy ache
• breast tenderness
• ‘spotting’ of blood
• dizziness

If you do happen to throw up after taking these, you’ve got to go get another dose.
The second form of emergency contraception is the IUD or coil. This has to be inserted by a doctor, and rests in your womb. It basically makes for a nasty environment that an egg doesn’t want to implant in. You can get it 5 days after unprotected sex, but once again; the sooner the better. This is less common, and is not generally done in clinics so much.

A note from -emmz-: you can only buy the morning after pill from a pharmacy if you're 16 or over. If you're under 16 then you need to go to your doctor, family planning clinic etc or some community pharmacies will run patient group directions where they can provide it to people under 16, and it's free! A lot nicer than the 25 quid it'll cost you otherwise!