For men and for women who aren't pregnant, the three antibiotics azithromycin, doxycycline, and tetracycline work just as well as each other to get rid of chlamydia. Antibiotics are drugs that kill bacteria (germs). There are many types of antibiotics.
You can take azithromycin (brand name Zithromax) as a single dose. If you're taking doxycycline or tetracycline, you'll need to take them for seven days.
Doctors usually prescribe doxycycline or azithromycin for treating chlamydia.
You can buy azithromycin over the counter, without a prescription, if you're over 16, and if you have tested positive for chlamydia and you don't have any symptoms. If you have symptoms, like discharge or itchiness, you should get them checked by a doctor. The medicine you can buy from pharmacists is called Clamelle. You'll need to buy a test kit first, also branded Clamelle, to check whether you have chlamydia.
Although you may find it convenient to buy treatment yourself from a pharmacist, it's likely to be more expensive than seeing your GP or going to a GUM clinic. You can expect to pay around £25 for the test kit and £20 for the medicine. If you go thorough the NHS, chlamydia testing and treatment is often free, although if you go to your GP you'll probably have to pay the standard prescription charge. Prescription charges vary depending on where in the UK you live.
The NHS screening programme is trying to make chlamydia testing and treatment more widely available, especially for younger people. It's aimed at people under 25. In England, if you'd rather not see your GP or go to a GUM clinic, you can get testing from some pharmacies, drop-in centres, universities, and family planning clinics. You can find out more at the NHS website (http://www.chlamydiascreening.nhs.uk) or by calling 0800 567123.
There's good evidence that antibiotics work for chlamydia. One summary of the research (a systematic review) included 12 good-quality studies (randomised controlled trials) that looked at taking antibiotics for chlamydia. The summary included a total of 1,543 people. It showed that about 8 in 10 to 9 in 10 people with chlamydia were cured after taking either azithromycin or doxycycline.  Other research has shown that tetracycline works just as well. 
The advantage of azithromycin is that it works after taking a single dose. But you'll need to wait seven days after having treatment before having sex again. That's to be sure the treatment has had time to work. With the other medicines, you need to wait until you've finished taking the tablets. 
The side effects of these antibiotics are usually mild.  The most common ones are:
Getting an upset stomach
Getting stomach pain
Occasionally, people taking doxycycline or tetracycline get a skin rash when they are outside in sunlight. Doctors call this photosensitivity.