A 1940s antihistamine remains a medical paradox, balancing life-saving relief with severe tissue risks.

Nature · Published 3 October 2026 Swipe more like this →

In short: Developed in the 1940s, promethazine is a versatile tool used for everything from allergies to morning sickness. Yet, this common medication carries significant risks, including severe tissue damage from improper injection and respiratory dangers for young children. Understanding its complex pharmacology reveals why it remains both essential and controversial.

Synthesized by Paul Charpentier and his team at Rhône-Poulenc in the 1940s, promethazine was designed to improve upon existing antihistamines like diphenhydramine. This chemical lineage eventually led to the development of chlorpromazine. Today, it is a global staple, marketed under dozens of names and used for conditions ranging from motion sickness and chemotherapy-induced nausea to severe morning sickness, for which it is the first-choice treatment in the UK.

The drug’s utility stems from its broad pharmacological profile. It acts as a strong H1 receptor antagonist, but also interacts with serotonin, dopamine, and muscarinic acetylcholine receptors. Recent research suggests it even functions as a non-competitive NMDA receptor antagonist. While these mechanisms provide its sedative and antiemetic effects, they also underpin its significant side-effect profile, which includes blurred vision, constipation, and cognitive impairment due to its high anticholinergic burden.

Safety remains a primary concern, particularly regarding administration and age. Because injectable promethazine is a potent vesicant, it can cause severe tissue necrosis and gangrene if it leaks into surrounding tissue or enters an artery, leading to catastrophic injuries like amputations. Consequently, the Institute for Safe Medication Practices advises strict dosage limits and careful site selection. Furthermore, its use is widely contraindicated in children under two years old due to the risk of respiratory depression and sleep apnea, and it is generally avoided in the elderly to prevent long-term cognitive decline.

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