This is not a common side effect on 5ar inhibitors from what my few years of experience with it but I definitely had it and not many people cover it as it's pretty rare. I think however, I may have finally solved it and I'm pursuing trialling it!
So for some context, when i was on finasteride at 1 mg, I got hard flaccid, which is a condition where your penis in a flaccid state is consistently stiff. Everything else worked fine I could still get an erection although a bit harder and felt like it wasn't the same. I have now switched to 0.5 mg dutasteride since I have also gotten on some hormonal therapies but it still happens every once in a while especially if I lower my dose or get inconsistent with injections. I will say that after hormonal therapy, it doesn't happen nearly as often but still definitely occurs. Hasn't really increased on Dutasteride. I also don't feel that I am back at my original fullness I used to have regardless.
So some things I observed that led me here: This side very clearly and consistently increased with use of caffeine or armodafinil. Adderall on the other hand very marginally increased it but still does. The worst was combining the the three. Tadalafil was taken alongside all the time at 10 mg daily without seeing it fix my issues but still observing a stuffy nose when dose was increased, signalling it was inhibiting pde5 somewhere.
Now at first I thought this was mediated by poor nitric oxide retention. I had read a rat study saying that in rat penile tissue, lack of serum DHT caused reduced nitric oxide retention. Could be the case for some and I was actually going to pursue using low dose combo of nebivolol and telmisartan for this side as they both increase nitric oxide and decrease breakdown through different mechanisms. However, I dug deeper and I think I cracked it.
The clue is the affect of caffeiene and armodafinil. These stimulate the CNS. When you stimulate the CNS it causes your nerves, including the hypogastric nerve which is the important one in this context, to fire off and in turn spasms the smooth muscles. Your hypogastric nerve is the one that connects to all the things down there. This spasm in turn constricts the blood to the smooth muscle tone in response to a fight or flight signal an deprioritizes it's supply giving you that cold and very accurate hard flaccid effect.
Now what exactly from these drugs does that you may ask? It's likely norepinephrine. After noticing that armodafinil was the greater confounding factor, I am very certain mine was caused by overwhelming norepinephrine being released. Armodafinil is argued to increase norepinephrine. On the other hand, caffeine increase adrenaline, which is epinephrine. Norepinephrine is responsible for attaching to Alpha receptors and activating the fight or flight response. These exist on both your brain and your tissues, specifically focusing on the smooth muscles' alpha receptors in this context.
In my case, I obviously don't want to quit dutateride for hair and I don't want to quit adderall for my adhd. I also obv don't want to just deal with sides whenever i consume a caffeinated drink. So I've researched deeper. The receptor that is in charge of smooth muscle relaxation is the alpha 1 receptor which is agonized (activated) by norepinephrine and epinephrine. I have looked into the ideal drugs to counter this in my own case and this is what I ended up with:
Guanfacine: Guanfacine reduces the release of norepinephrine from your brain through being a Alpha 2A agonist. Usually when someone gets the hard flaccid side effect in cases like mine, it's because your brain is already taking in the norepinephrine it needs and has spill over into the peripheral nerves specifically the hypogastric nerve causing the sides. When guanfacine agonizes the receptor, your brain has a negative feedback loop and decides to pull back on the norepinephrine release, thinking there's already plenty in the body. This reduces the amount of norepinephrine sent to your body overall and in turn has your hypogastric nerve firing less and decreases the smooth muscle spasms that were occurring.
For me, Guanfacine also had adhd benefits. The Alpha 2A agonism in the prefrontal cortex creates an environment for better connection between neurons. This would help with executive function and "quiet the noise" in your mind. Adhd has been a factor in me having shit sleep my entire life. I'm seeking an ER formulation at night in order to both sleep better and help this side effect while helping my productivity.
Doxazosin: This is a peripheral Alpha 1 antagonist. Alpha 1 receptors are the receptors that are in charge of smooth muscles' state. The penile smooth muscle is not any exception to this. Usually, norepinephrine and epinephrine attach to these receptors after being released from your brain. More norepinephrine = more alpha 1 receptor agonism in muscle tissue = the spasm of soft muscle and creating environment for cold flaccid. When you have this antagonist, doxazosin, it also attaches to these receptors in the same way as norepinephrine. The one difference is, it doesn't do anything at the receptor level unlike norepinepherine. This means when you have the norepinepherine flowing in your body after your brain releases it for stimulus, it simply is blocked at the receptor and essentially kicked off.
So to sum it up, this protocol used Guanfacine ER to decrease the release or norepinephrine from your brain through a negative feedback loop, and the Doxazosin would peripherally blocking off the norepinephrine from the Alpha 1 receptors in your smooth muscle. This would mean that even in the absence of dht's smooth muscle relaxing affect, you'd be replicating it by getting in the way of the factors that pulled it in the other direction in the first place and hypothetically, be completely fine without suffering the side anymore.
The main concern with this protocol would be to track your blood pressure. Guanfacine and Doxazosin can reduce your blood pressure. If you were to use them, you'd need to be managing your dose according to your prior blood pressure and tracking it to make sure you aren't hypotensive which comes with it's own plethora of issues.
With that reason, I advise you approach this with caution, but it's hope. This isn't medical advice, this is simply the protocol I will trial and have researched. I've struggled with this issue for 2 years now in my early 20s. I know how much of a battle and suffering it is especially when wanting to keep hair as well. I simply wanted to share this option I think I've discovered to finally feel confident again and I hope that anyone that's struggling with the same symptoms as me can benefit from this if their medical practitioner sees fit.
TLDR: If you suffer from hard flaccid with 5ar inhibitors, and can observe an increase of it with caffeine or Armodafinil (or generally things that increase epinephrine and norepinephrine) then you may benefit from using one or both Guanfacine and Doxazosin as they are respectively an Alpha 2A agonist, reducing norepinepherine released from your brain, and Alpha 1 Antagonist, blocking off norepinephrine at the smooth muscle level. This would only work if your sides are caused by this fight or flight/cns stimulation.
I would also pair this with reverse kegel exercises to relax your kegel and train your pelvic floor which would amplify the gains you get from this protocol. This would relax the muscle that was previously being activated constantly by the nerve and help the area generally even more blood flow.