Explained: What painkillers do to the human body

Here's what prescriptions and over the counter painkillers work in your body.
Here's what prescriptions and over the counter painkillers work in your body. Photo credit: Getty

Have you ever wondered what happens to paracetamol when you pop some for a headache, or the ibuprofen you swallow when you have the flu?

Here Newshub explores how common over-the-counter and prescription painkillers work in your body, and what side effects they may have.

Paracetamol: Panadol, Advil (over-the-counter)

This painkiller is used to treat any type of acute or mild short-term pain.

How it works

A 2018 study found paracetamol provides relief due to the indirect activation of your endocannabinoid system (ECS). ECS is a complex cell system which exists within your body, and responds to CBD and THC, found in cannabis. Scientists are still trying to fully understand the role of ECS, but know it regulates a range of functions, such as sleep, mood, appetite, memory, reproduction and fertility. ECS is also known to heighten the effects of whatever substance someone has taken, such as paracetamol.

A picture of a Paracetamol packet, listing the common side effects.
The common side effects of Paracetamol are listed on the packet. Photo credit: Supplied

Long-term/short-term side effects

Paracetamol may cause some rare side effects and you should seek medical attention if they occur. These side effects include blood in your urine or stool, fever, rash and yellow eyes or skin.

Taking more of the recommended amount of paracetamol per day can cause liver damage. It is advised people should use as little as possible.

Professor Andrew Moore of Oxford University says the greatest concern of paracetamol use is accidental overdoses.

"[People think] I have a headache, so I'll take some Panadol, I have a cold I'll take a cold product like Lemsip. People don't necessarily look at the small print," he told the Guardian in 2015.


NSAIDs: Ibuprofen, Nurofen (over-the-counter)

Non-steroid anti-inflammatory drugs (NSAIDs) are used to reduce mild-to-moderate pain and inflammation. They can be used for short-term pain such as an injury and a flare-up of pain symptoms.

How it works

NSAIDs work by blocking enzymes and reducing prostaglandins, a hormone-like substance contributing to inflammation in the body. The inflammation presents itself as swelling, fever or pain.

A picture of an Ibuprofen packet, with the common side effects listed.
The common side effects of Ibuprofen are listed on the packet. Photo credit: Supplied

Long-term/short-term side effects

NSAIDs can have harmful side effects if used for a long period of time. These include decreased kidney function, increased blood pressure, stomach ulcers, heart attacks and strokes. However, these side effects are mostly seen in people who are already suffering from serious medical conditions.

Nerve painkillers: Anticonvulsants and antidepressants (prescription)

This group of prescription painkillers have been used for depression and epilepsy in the past, but are now seen to be beneficial as a painkiller in lower quantities.

How it works

These painkillers relieve pain from damaged nerves. This damage causes pain signals to misfire, making the brain think the body is in pain, when it is not. The medication aims to change these signals, reducing pain overall.

Long-term/short-term side effects

These drugs can have a range of significant side effects, with some being very serious. The most concerning long-term side effects include pancreatitis, hepatitis, hallucinations, loss of consciousness and respiratory failure.

Opioids/opiates (prescription)

Opioids are prescription painkillers used to treat moderate-to-severe pain. Not all opioids are the same, differing in what they do to your body, and how they move throughout your body.

Opioids in New Zealand are divided into two different groups - weak and strong. Weak opioids are prescribed when other painkillers have not worked for the patient - they include tramadol, dihydrocodeine and codeine.

Strong opioids may be trialled when weak opioids have not had any beneficial effect. Strong opioids include morphine, oxycodone, methadone, fentanyl and pethidine.

How it works

Although different opioids can have different effects, they all control pain relief by binding to your body's opioid receptors. Opioids activate the receptors located in the brain, spinal cord, and other organs that are associated with feeling pain or pleasure. After taking opioids for an extended period of time, the brain adapts to the drug. This diminishes the brain's sensitivity, making it harder for a patient to feel any pain relief or pleasure from anything except the drug.

Long-term/short-term side effects

Many opioid users report unpleasant side effects, such as constipation, nausea, sleepiness and dizziness. Long-term use of opioids can have serious consequences, such as hypogonadism, adrenal insufficiency and abnormal pain sensitivity. A major concern about opioids is around dependence. A patient can suffer from substance use disorder even if using opioids as they were prescribed. Dependence can appear within just one month of use. In high enough doses, opioids can lead to respiratory failure, coma and death. Prescription fentanyl can reportedly be up to 50 times more potent than the illegal drug heroin. A teaspoon of powdered fentanyl can kill up to 1000 people.

Newshub has cross-checked the medical analysis in this article with Dr Samantha Murton, President of The Royal New Zealand College of General Practitioners.