(As part of FIT's #DecodingPain series, we pull apart the many layers of pain–the sensation, the causes, the stigma, and the treatments. In this article, we explain painkillers.)

For as long as sentient creatures have existed, so has the mysterious sensation of pain. And as long as humans have existed, so has the quest to find relief from pain.

From rituals, ice, willow bark, to opium, and even electric Eel bites, pain relief has a long history of hit and misses before it was synthasised into little pills and bottled in the early 19th century.

Chances are we've all popped a painkiller at some point in our lives. But how much do we really know about them?

How do painkillers work? Why are some types addictive? Do different types of pain need different kinds of painkillers?

FIT breaks it down.

Before we dive into it, here is a quick rundown on how painkillers work.

Painkillers can be broadly classified into 3 categories: Opioids, NSAIDs and Acetaminophen (paracetamol).

Opioids are painkillers derived from opium–an extraction from poppy plants that has been used for centuries for both medicinal and recreational purposes all over the world.

Opioids contain a substance called morphine that bind to the opiate receptors of our nerves where endorphins (endogenous morphine) would typically bind, creating the same analgesic effect that the latter creates.

NSAIDs on the other hand are anti-inflammatory painkillers. This means they not only help dull pain but also reduce swelling and fever.

They work like this: NSAIDs block the production of prostaglandins–lipids produced by the body at the site of an injury that causes pain and inflammation

Acetaminophen like paracetamol are used in mild to moderate cases. These are far safer than other types of painkillers, even for use in children and the elderly, and are available over the counter without a prescription.

Some painkillers are combinations of NSAIDs and Acetaminophens.

"Yes," says Dr Anshu Roghatgi, a neurologist at Sir Gangaram Hospital, Delhi. "Different pain requires different kinds of an analgesics (pain relievers) and that is very important to identify."

So pain that is a result of inflammation or injury like a fracture or body ache from a viral infection can be soothed with the help of NSAIDs whereas neuropathic pain like chronic pain would require opioids.

"Deciding on the type of painkiller to prescribe will have to rely on clinical sense, judgement and what your diagnosis is," he says.

It will have to start by looking at what is causing the pain first, explains Dr Roghatgi. "We'll have to first see if it being caused by an injury, the nerves."

"At the end of the painkillers are not cures, and the underlying cause for the pain needs to identified and treated." he adds.

Some of the most widely used prescription opioids are: Codeine, Morphine, Oxycodone, and fentanyl.

What makes opioids such a widely used painkiller?

"Opioids can work in all kinds of pain. They can help relieve muscular skeletal pain, neuropathic pains and even visceral pain," says Dr Roghatgi.

They are especially useful in tackling chronic, long term persistent pain like in the case of cancer.

It is this versatility of opioids in treating different kinds of pain, on top of aggressive marketing by pharmaceutical companies in the 1990s that led to opioids being prescribed indiscriminately.

They were touted as the next big thing in medicine, and the safest painkiller.

But by the time the world realised that this was far from the truth, it was already too late.

Today, USA in particular, faces a massive epidemic of opioid addiction.

The downside

What makes opioids so addictive?

As it happens, opiate receptors are found all over the body, and there is no real way for the drug to identify which receptors to bind to.

Opioids, therefore, also bind with receptors on neurotransmitters that control the release of dopamine–our body's feel good hormone.

This is what leads to the opioids' effect of calmness, relief and euphoria.

But over time, the body builds tolerance to these drugs as it naturally works harder to maintain balance by inhibiting the flow of dopamine.

This is the reason people experience withdrawal symptoms like excessive pain, depression and physical sickness.

This is also the reason opioids are highly controlled drugs and are only available with a prescription.

Moreover, it is also suggested that women are more prone to get addicted to prescription opioids.

One likely reason for this is that women are more prone to chronic pain, and so have been prescribed opioids in greater numbers than men.

NSAIDs are the most prescribed pain relievers in te world. A reason for this is that they are far safer than opioids in that they do not lead to dependancy.

These are basically anti-inflamatory drug, explains Dr Rohatgi, "All of them act on different parts of the inflammatory process."

But, "NSAIDs won't work for chronic or neuropathic pain," he explains.

Some NSAIDs, unlike opioids, can be bought without a prescription.

Some of the most common types of NSAIDs are ibuprofen, diclofenac, and naproxen.

Asprins, popularly known in India by the brand name Disprin, is typically only considered an NSAID in higher doses.

The Downside

Though NSAIDs are far safer than opioids and not addicitve, they have their own set of side effects, especially when taken in high doses over a long period of time.

Dr Gagandeep Singh, neurologist and professor at Dayanand Medical College, Ludhiana, also speaks of 'medication overuse headaches'.

This especially occurs in those with migraines and other headache disorders, who use excessive NSAIDs for pain relief.

"A vicious cycle gets created," explains Dr Singh.

He explains, " In case of chronic headaches like migraines it happens so that over time NSAIDs could start becoming less effective or increasing the intensity of the pain when the affect of the medicine ebbs."

Thic causes a person to double down on the pills, which can further increase headaches.

According to the American Migraine Foundation, MOH may be accompanied by nausea, anxiety, irritability, asthenia, restlessness, and difficulty concentrating.

Depending on the type and intensity of the pain one is suffering from, different types of pain kills may be needed.

Of all the analgesics, paracetamol is the the safest with the lease possibility of side effects and long term damage.

Having said that, all painkillers must be used judiciously. Always take painkllers under a doctor's supervision.