An analgesic is a drug used for the control or relief of pain. Analgesics have been used since approximately 400BC, in the case of willow tree bark, which was noted by Hippocrates as being "useful for relieving the pain of childbirth" when chewed. The active ingredient was isolated and purified, and is now referred to as aspirin.
Analgesics differ from anesthetics in that anesthetics are usually given because pain is anticipated, and analgesics are usually given because pain is already being experienced.
Analgesics are often colloquially referred to as painkillers. They can be taken orally (in tablet or liquid form) such as acetaminophen/paracetamol, ibuprofen, and aspirin; they can also be applied to the skin directly in the case of ibuprofen gel, which is usually used for joint pain), and a few others.
Although some people prefer one particular analgesic over another, and some have no marked preference, care should be taken to choose the appropriate analgesic if you are experiencing post-modification pain.
How analgesics relate to modification
Some people believe that the pain of a modification must be overcome naturally, without the aid of analgesics. Others feel that they should take advantage of modern medicine to make the process more bearable. No one should be judged for choosing either path; it remains a matter of individual choice.
For those who choose to use pain relief, even if only for a short period, it's important to choose a suitable analgesic, as some may complicate the healing process. The most notable example of this is aspirin, because it thins the blood, which may worsen bleeding and encourage bruising, as well as irritating the stomach lining. Safer options have been mentioned above.
As with all drugs, it is wise to inform your piercer or tattoo artist if you are taking any drugs, whether therapeutic or recreational, or are intending to take any medication afterwards. He or she will advise you on a sensible choice of analgesic. Your pharmacist should also be able to give you sound advice on choosing the analgesic that's right for you.