I don't think that addiction and withdrawal mechanics are a good idea for this. You want to make sure it's still possible for the party member to have a change of heart and realize the error of their ways and stop drinking the potion. Lycanthropy is a set of strong buffs: the damage immunities, the strength and speed increases, and the bonus to AC. There should be a strong disincentive for using this potion, otherwise it just won't make sense to not use it.
I think the best disincentive, and one that's traditional for lycanthropy, is losing control of your character. When a party member first takes this potion, have them make a DC 5 Wisdom saving throw. If they succeed, they get all the buffs for the time period, and they feel like a bad-ass. They'll want to use it again. Every time they use the potion in the future, increase the DC by 5 each time. If they fail their saving throw, they lose control of their character for the time period. As DM, have the party member who is now an out-of-control werewolf attack the party members.
I'd encourage you to take control of the character entirely during this time period. If you let the party member who drank it control this bad-ass werewolf attacking the party, the other players might more strongly associate the actions of the out-of-control werewolf with the player who chose to drank the potion. They'll already be pissed. At least make it clear that you are the one in control when their character is attacking the party.
You haven't mentioned how long the potion's effects last for. I'd recommend 1 minute. That's long enough to be useful for your villainous NPCs to get the buffs for an entire combat, but it's short enough that if a party member loses control, the party can wait it out.
Obviously take all this advice with a grain of salt. I'm very much a fan of the idea that lycanthropy causes people to lose control (so much so, that in my current homebrew setting, lycanthropy is literal possession by a demon). I'm not totally sure what the history of lycanthropy in Eberron is, but my understanding is that it's even more reviled than in the Forgotten Realms. You might find this blog post by Keith Baker (the creator of Eberron) helpful for your campaign: http://keith-baker.com/dragonmarks-lycanthropes/