Love is a drug as powerful as morphine and any painkiller

Passions triggered by the early flushes of a relationship block physical pain in a similar way to painkillers and drugs, a study has shown.

Scientists in the US tested the theory on 15 male and female university students who were in the passionate early stages of a love affair.

They were shown photos of their partners while a computer-controlled heat probe placed in the palms of their hands delivered mild doses of pain.

At the same time, the students had their brains scanned by a functional magnetic resonance (fMRI) imaging machine.

The study showed that feelings of love, triggered by seeing a photo of one's beloved, acted as a powerful pain killer.

Focusing on a photo of an attractive acquaintance rather than a relationship partner did not have the same benefit.

The scans revealed that the effects of love could be compared with those of morphine and cocaine, both of which target the brain's "reward centres".

Dr Sean Mackey, the study leader and head of the Division of Pain Management at Stanford University Medical Center in California, said: "When people are in this passionate, all-consuming phase of love, there are significant alterations in their mood that are impacting their experience of pain.

"We're beginning to tease apart some of these reward systems in the brain and how they influence pain.

"These are very deep, old systems in our brain that involve dopamine – a primary neurotransmitter that influences mood, reward and motivation."

The scientists recruited Stanford students who were in the first nine months of a romantic relationship.

"We intentionally focused on this early phase of passionate love," said Dr Mackey.

"We specifically were not looking for longer-lasting, more mature phases of the relationship.

"We wanted subjects who were feeling euphoric, energetic, obsessively thinking about their beloved, craving their presence.

"When passionate love is described like this, it in some ways sounds like an addiction.

"We thought, 'maybe this does involve similar brain systems as those involved in addictions which are heavily dopamine-related'."

Dopamine is one of a family of brain chemicals that transmit signals between neurons.

It is at the heart of the brain's "reward" system – helping us to "feel good" when enjoying pleasurable experiences.

Dopamine pathways are closely associated with addiction and the deep-level pain relief induced by morphine and other opioid drugs.

The study found that word-association distraction tasks also reduced pain but in a different way.

Study participants were given mental challenges such as thinking of non-ball sports to take their minds off pain.

The aim was to ensure that love was not simply working as a distraction.

The scientists found that both love and distraction combat pain, but they act on very different brain pathways.

Dr Jarred Younger, the co-researcher also from Stanford, said: "Love-induced analgesia is much more associated with the reward centres.

"It appears to involve more primitive aspects of the brain, activating deep structures that may block pain at a spinal level – similar to how opioid analgesics work.