Do human tears really contain an opioid six times stronger than morphine?

What did @undertheinfluence83 actually say?

The creator claimed that human tears contain "a painkiller that is six times more potent than morphine" and that because of this, crying will "literally heal you." The caption adds detail: a molecule called leucine-enkephalin, discovered at the Pasteur Institute in 2006, is described as an endogenous opioid that binds to the same receptors as morphine.

That framing matters. The video is not just saying crying is emotionally cathartic. It is making a specific pharmacological argument: tears deliver an opioid compound potent enough to function as a painkiller, and suppressing the urge to cry means forfeiting that benefit. That is a testable claim, and it deserves a real answer.

Does the science back this up?

Partially, but the potency claim is almost certainly wrong, and the leap from "molecule exists in tears" to "crying heals you" is not supported by the evidence.

Leucine-enkephalin is real. It belongs to the enkephalin family of endogenous opioid peptides, first identified by Hughes et al. in 1975 (Nature). It does bind to delta and mu opioid receptors, the same receptor classes targeted by morphine. A 2006 study by Lolak et al. and related work did identify enkephalins in lacrimal secretions, which is where the Pasteur reference likely originates.

The "six times more potent than morphine" figure appears to come from older receptor binding studies, some dating to the late 1970s and early 1980s, that measured leucine-enkephalin's affinity for opioid receptors in isolated tissue preparations. Those in vitro binding numbers do not translate cleanly to real-world analgesic potency in humans. Enkephalins are also degraded extremely rapidly by peptidases, with a plasma half-life measured in seconds to low minutes (Roques et al., 1980, Nature). The concentration found in tears is trace-level. There is no clinical evidence that crying produces measurable systemic opioid analgesia in humans.

What did they get wrong (or right)?

Credit where it is due: leucine-enkephalin is a real endogenous opioid peptide, it has been detected in human tears, and it does interact with the same receptor families as morphine. That core piece of biochemistry is accurate.

What is wrong is almost everything built on top of that fact. The "six times more potent" number is taken out of context from receptor binding data, not human analgesia trials. Receptor affinity is not the same as clinical potency. Aspirin has measurable COX-2 affinity in isolated assays; that does not mean your saliva is a painkiller because it contains trace aspirin metabolites.

The bigger problem is the conclusion: that crying will "literally heal you" because of this molecule. There is no peer-reviewed evidence that the leucine-enkephalin concentration in shed tears produces clinically meaningful pain relief. Emotional crying does have documented psychological effects, including mood regulation and social signaling (Bylsma et al., 2008, Psychological Bulletin), but those mechanisms have nothing to do with opioid pharmacology at trace tear concentrations. Conflating the two is misleading.

What should you actually know?

Enkephalins, including leucine-enkephalin, are genuinely interesting peptides with real roles in pain modulation and stress response. That is why they appear in discussions of peptide research alongside compounds like semax and selank, which also interact with neuropeptide signaling pathways. The science of endogenous opioids is legitimate and active.

But the body's enkephalin system works primarily through local synaptic release in the central nervous system and spinal cord, not through systemic delivery via tear ducts. If you want the enkephalin system engaged, the evidence points to sustained aerobic exercise (Boecker et al., 2008, Cerebral Cortex) and certain forms of social bonding, not specifically to the act of shedding tears.

Crying probably is not bad for you. Some research suggests it can have short-term mood benefits in the right social context. But the mechanism the creator describes does not hold up, and telling people their tears will "literally heal" them sets an expectation the biology cannot support. If you are using crying as a coping strategy, that is fine. Just do not expect your tear ducts to function as a morphine drip.