Today let’s decode antihistamines, and not just as a hay fever treatment, but as an emerging intervention for chronic illness. If you have Long COVID, PoTS, ME/CFS or MCAS you will have heard people in patient communities talking about antihistamines. The evidence behind the conversation is more substantive than most clinicians currently acknowledge and a significant new trial published in The Lancet on 8 July 2026 adds the most rigorous data yet.
What are antihistamines and why might they matter ?
Histamine is a chemical produced by your immune system (particularly by mast cells, the cells that trigger allergic responses). Most people (me included) encounter antihistamines at some stage, such as hay fever tablets, drugs that block histamine receptors and reduce the sneezing and itching that accompany allergic reactions.
Though histamine does far more than trigger allergies… It regulates blood vessel dilation, influences heart rate, controls gastric acid and acts as a neurotransmitter in the brain - playing a central role in immune activation and inflammation. In conditions where mast cells are overactive, such as in mast cell activation syndrome (MCAS) or where histamine clearance is impaired (perhaps the case in a subset of Long COVID, ME/CFS and PoTS patients), histamine can drive a wide range of symptoms that look nothing like a classic allergic reaction: brain fog, fatigue, orthostatic intolerance, gastrointestinal problems and cardiovascular symptoms.
This is why antihistamines have been appearing on the radar of researchers and clinicians working in these conditions. It is not that histamine is the root cause. It is that for some patients, it appears to be a significant amplifier of symptoms through a mechanism that antihistamines can partially address.
The new Lancet trial
Published in The Lancet Infectious Diseases on 8th July 2026 and funded by the NIHR, this is the largest trial of antihistamines in Long COVID to date. Led by UCL and UCLH, it recruited around 800 adults in England with Long COVID. Patients were randomly assigned to one of four groups: 1) specialist care alone, 2) specialist care plus a combination of famotidine (40mg; usually used for acid reflux) and loratadine (10mg), 3) specialist care plus colchicine (an anti-inflammatory used to treat gout) or 4) specialist care plus rivaroxaban (a blood thinner).
