Antihistamines blunting training adaptations

My training has been super solid this year but performance has been consistently weaker than last season despite very similar diet, routines and so. For example, FTP is sitting at 310-315 rather than 320-330 and 5, 10 & 20 minute power is off by a similar percentage.

One thing that occurs to me is that last August - per my GPs advice - I started taking Phenergan at night to get to sleep. I’ve used it religiously since then as having a night off is a recipe for no sleep. I know there’s something not-quite-right about it but the GP assures me it’s safe and I’ve just continued as it works so well.

Googling around, it seems there is some evidence that antihistamines can “blunt training adaptations” (The weird connection between exercise and antihistamines — study) which is pretty wild…

Starting last night I will give the Phenergan a break and let’s see if anything changes over the next block. I will be a bit sleep-deprived for a few nights but happy to give it a go.

Does anyone else have experience with long-term antihistamine use having an impact on performance?

If you don’t need the antihistamine to combat allergies there are other options to help with sleep. Personally I’m prescribed Propavan with positive results. If you’re concerned you should talk to your doctor.

Edit: added this link.

Just off the cuff and from memory, but I think the dosage of anti-h in the study is very large compared to what a normal person would take.

Are you able to try melatonin? A little goes a long way as long as your sleep hygiene is good.


Thanks for the responses folks. That study did have quite high dosages but I am just struggling to find something that explains the performance dip. I will report back on how I go after 4-6 weeks off the meds.

Also thank you for suggesting melatonin and Propavan. I will take a look if I can’t get my sleep back on track with good hygiene.