It's that time of year when Christmas chaos, family dinners and too much office party wine add up to a visit to the pharmacy for some pain relief.

Whether the holiday season gives you a tension headache, migraine or backache, there seems to be a box of painkillers targeted specifically for your needs.

But how can a painkiller that you swallow and digest then direct itself to the specific part of your body that hurts?

If you live in Australia, that question has been answered and the science says it can't.

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The Australian painkiller section is about to get much smaller after drug company Reckitt Benckiser was ordered to pull its site-specific painkillers off the shelves.

The company had admitted products marketed for specific types of pain such as tension headache and period pain were identical to their general product even though they cost twice as much.

The court found these site-specific Nurofen products all contained exactly the same active ingredient, 342 milligrams of ibuprofen lysine (equivalent to 200mg of standard ibuprofen) and that none were any better at treating one type of pain than other products in the range.

It is not the only company to do this. A simple scan along the painkiller shelves reveals a series of site-specific labelled painkillers with premium pricing containing the same active ingredients as the standard cheaper version - resulting in the biggest pain being in your pocket.

Painkillers work by blocking enzymes everywhere in the body, not just in the area causing pain. Injury and infection result in high levels of chemicals called prostaglandins being produced locally which cause symptoms including redness, swelling, heat and pain more broadly known as inflammation.

This is an important part of the body's normal healing process and the prostaglandin inflammatory response makes us aware that there is a problem preventing us from further injury by immobilising a joint through swelling or indicating we need to remove our hand from a hot surface.

This natural response, however, can sometimes lead to too much or chronic prostaglandin production, making us feel pain when we don't need or want to feel it any more.

The likes of ibuprofen, paracetamol and aspirin work by inhibiting the synthesis of prostaglandins by interfering with the action of an enzyme called cyclo-oxygenase which catalyses the conversion of a compound called arachidonic acid into prostaglandins. Fewer prostaglandins means less pain and inflammation.

Ibuprofen and aspirin reduce both pain and inflammation, whereas paracetamol reduces pain only.

The price difference between generic, brand name and site-specific painkillers can be up to 10-fold, but there is no reason to expect one product to be more effective than another. Where the products do vary is in how long it takes a tablet to dissolve in the stomach once ingested due to liquid and solid forms of capsule with some having faster-dissolving coatings on the surface.

This doesn't make them work better, it just makes them absorb and act more quickly.

As New Zealand is currently keeping site-specific painkillers on our shelves, it's totally up to us, the consumer, to make an informed decision. Science says the only difference is how much you want to pay.

• Dr Michelle Dickinson, also known as Nanogirl, is an Auckland University nanotechnologist who is passionate about getting Kiwis hooked on science.