Do antihistamines increase or decrease muscle soreness?

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Whether it’s ice baths, compression garments or antioxidants, the evidence for many supposed muscle soreness cures is slim at best. So a study that seemingly does offer some help is a surprising and welcome sight - especially when the “cure” it offers is totally unexpected.

A new study in the Journal of Applied Physiology, by Matthew Ely and his colleagues at the University of Oregon, tests the idea that taking a single dose of antihistamine medication can protect you from delayed onset muscle soreness (DOMS) after a hard workout. And on the surface, the results are eye-catching: the control group was 19.3 percent weaker the day after the hard workout, while the antihistamine group was only 7.8 percent weaker.

A (partial) cure for DOMS, at last? Not quite. On closer examination, the results are more complex, and tie into a larger debate about the balance between recovery and adaptation.

Some background: after hard exercise, blood flow to your muscles remains elevated for several hours. It’s thought that histamines (part of your body’s immune response) play a role in triggering this post-exercise blood flow, which may be linked to inflammation and subsequent repair of damaged muscle.

So the researchers’ initial hypothesis was that blocking histamines with an antihistamine would reduce post-exercise blood flow, reduce inflammation... and increase muscle damage and DOMS.

To test the idea, they had 12 volunteers run downhill on a 10-percent gradient for 45 minutes after taking a combination of fexofenadine and ranitidine, and 12 volunteers do the same thing without the antihistamines. Then they measured a bunch of outcomes, including blood flow, inflammatory markers in the blood, pain sensitivity, perceived soreness and strength for the next three days.

The first hypothesis appeared to be correct - blood flow to the legs an hour after exercise was reduced by 29 percent in the antihistamine group.

Unexpectedly, there were no differences in markers of inflammation. This is tricky to interpret, because it’s possible there may have been local differences in inflammatory response in the legs that didn’t show up “systemically” in the overall levels circulating throughout the body that they measured in the blood.

However, there were big differences in levels of creatine kinase, which is generally used as a marker of muscle damage. This seems to support the idea that blocking histamine receptors resulted in increased muscle damage, as expected.

That’s why the “headline” result - that antihistamines resulted in less perceived discomfort and greater preservation of strength - is so surprising.

So what’s going on here? There are a number of possible explanations, and the short answer is that no one knows for sure. One intriguing possibility, supported by previous research, is that the nerve fibres that carry signals of muscle pain back to the brain have histamine receptors that alter the sensitivity of the nerves - so the antihistamines make you feel less pain and soreness, regardless of the actual state of your muscles.

If so, the researchers point out, that makes antihistamines a double-edged sword. In day-to-day life, they may make your more prone to muscle soreness, and potentially to more serious forms of muscle damage as well. A few previous case reports have, in fact, linked cases of rhabdomyolysis (a breakdown of muscle tissue resulting in harmful proteins released into blood) to previous antihistamine use. So, while you’d certainly continue to take them if you have bad allergies, you wouldn’t be tempted to seek them out in search of a training boost.

On the flip side, if you have back-to-back competitions (“the track and field decathlon, or the multiple qualifying rounds of various athletic competitions” are the examples the researchers give), you might get an edge by taking an antihistamine to reduce next-day soreness and strength loss. (I’ll leave the ethics of this for another day.)

This duality is central to an ongoing debate surrounding all sorts of recovery aids, from ice baths to anti-inflammatories, that goes beyond the simple question of whether they work: if you interfere with the recovery process to make yourself feel better soon, do you also risk delaying the repair or adaptation processes? It’s a short-term versus long-term trade-off.

My read of the current evidence is that the answer depends on the circumstances. It’s possible to interfere with training adaptations (e.g. with high doses of antioxidant supplements), but there are probably also situations where hard-training athletes can use all the recovery help they can get. Right now, we don’t know enough to reliably say which is true in any given situation.

So where does that leave us? I don’t recommend rushing out to get allergy medication. As a more general principle, my feeling is that your routine training, if it’s following a gradual and logical progression, shouldn’t induce terrible muscle soreness on a regular basis. And if you generally alternate hard and easy days, it shouldn’t be a problem to occasionally run through a little mild soreness on easy days after the killer session the previous day.

An ounce of prevention, in other words, is worth a pound of potentially counter-productive and side-effect-laden cure.