So you've brewed up a potion that turns relatively normal fighters into horribly sadistic amoral monsters with incredible strength and regenerative powers, and you're wondering why the Jarls are looking at you funny? They've probably been down this road before, and they know it doesn't lead to a pretty place.
Modern historians are far from convinced as a whole, but there are certainly several of them who believe that Berserkers used various drugs to evoke the battle fugue. No doubt there were some who could do it on pure rage and blood lust, but for many of them the process may have been helped along with anything from henbane to fly agaric - both of which have some pretty nasty side effects, like vomiting blood and, you know, dying. It might get you through a battle, maybe two, but the accumulation of toxins will make for an excruciating last few days of your life... if you're lucky and don't end up living through it, weakened and broken.
In more recent times all sorts of pharmacological advances have been made, including such wonders as amphetamines which pretty much all modern armies have used at some point to combat sleep deprivation during long operations. Doesn't take much to understand why meth-heads shouldn't be flying planes or holding rifles, but during Vietnam over 225 million 'pep pills' (dextroamphetamine) were consumed by the US armed forces. The German soldiers on the front lines of WW2 consumed over 35 million methamphetamine pills, and they were working on a new battle drug - a mix of cocaine, meth and oxycodone - right before the war ended. And while armies probably never used it, Phencyclidine (aka PCP or Angel Dust) is a hell of a combat drug.
And now you're peddling a magical potion that makes PCP look like decaf? Uh, not for me thanks. I'm good.
The problem with performance enhancing magic is that there's almost always a price to pay. If you're lucky the price is up-front in terms of cost of manufacture: expensive components, lots of mana, maybe a few animal sacrifices. But more likely the price is going to come from the person using it.
First, the psychological price. The human mind is a delicate thing, and your potion rips into it; suppressing some parts, strengthening others and maybe rewriting whole aspects to make you the perfect sociopathic killer. And when the potion runs its course and the magic is spent, it's not going to linger long enough to put your fractured psyche back together again. A hit like that is going to leave permanent scars, and most likely the result will be complete psychosis.
Physical enhancements can also be expensive in some fairly nasty ways. Is the potion actually beefing up your muscles like Bane? Or are you just being flooded with combat hormones, like adrenaline and so on? Perhaps it's just letting you access the sort of hysterical strength that lets mothers lift cars off their children, or suppressing your pain reception so you don't notice how badly torn up you muscle fibers are getting every time you move.
But surely healing is a good thing, right? Well, that depends on how it works. Divine healing spells, healing potions and the like are definitely positives. They draw energy from outside of you, or from their own ingredients, and use that energy to fix what's broken. Your potion on the other hand forces the body's own healing processes into overdrive, constantly fixing what's being broken by your own physical excesses. In a magical context that probably means that you're using your own essence to power the regeneration, and essence is quite literally life. An hour of enhanced power is 5 years off your life, and it shows. Young, healthy warriors walk off the battlefield as broken down old men. Mature warriors may run out of time in the middle of the battle.
On a more... biological level, if you've ever experienced adrenaline fatigue you'll know that it's not something you ever want to experience again. It's unpleasant to say the least. Adrenaline hangovers are about as much fun as waking up in a ditch after a 3-day bender. Your potion is 10 times worse, and the hangover itself will kill half of the people who use the potion.
And yet there will be those who will chose to sacrifice themselves for the good of the tribe. The problem isn't getting them to take it, it's to stop them from taking it until there is quite literally no other option. You know, and they know, that there's about a 1-in-10 chance they'll survive the experience as anything other than an insane, broken wreck. They're likely to die outright or have to be put down for the good of the tribe. It's a desperation move, a last slim chance to rescue some shreds of victory from the howling maw of certain defeat... or to take the bastards down with you.
Use it wisely, and pray the gods forgive you.