"Poisoned" == non-zero toxicity level (i.e. under the effects of at least one potion), from what I understand.

If you choose to take Impregnation, the ability that increases the effects of mutagens, then don't use mutagens until you get it, as the increase is only applied to mutagens as they are used, already used mutagens are not increased unfortunately.

That's what I thought and it would make sense for the Alchemy path. The manual says "intoxicated" so I was a bit confused, especially since TW1 had talents that were only active when Geralt was drunk.

The game uses the terms "Poisoned" and "Intoxicated" interchangeably.

Tips?
Use elixirs, stay intoxicated for as long as you can because alchemy path IMO is the best one available (well, alchemy+sword or magic is quite nice too ^^)

Drawbacks?
Draug fight... afaik theres no way to be poisoned after all of those cut-scenes and minigames when you play as undeads... sooooo you're without awesome buffs but you still have some nice bombs in your pocket ;)
It might be annoying but its not a thing that could stop your progress when you know what you're doing (even on insane).

Yeah, I ended up dropping a few points in Magic my first go round to boost the two signs I used most, so I'll probably do the same this time around in anticipation of the Draug fight and some other times where I anticipate my potion timer running out during dialogues. That really seems to be the only drawback to choosing that build.

If you can roll good enough - one/two point/s in feet work is all you need IMO. Quen is well, too OP for me - even w/o upgrades it takes all damage away - I still use it but I'm not upgrading it on alchemy build (mage build is other story when you get adrenaline when you get hit with quen on)
For Draug - just don't forget that alchemy is not only about elixirs but bombs and traps too :)

As for potion duration... you can use a mod that extends their duration... but well thats up for user as some sees that like a cheating others like fixing oblivious designs flaws.

I've seen that mod, and it does feel a bit like cheating since the potion timers only negatively affect the game when they're left running during dialogue or mini-game sequences. CDPR should definitely patch that as it seems like it nerfs the class.

I just completed my Alchemy tree playthrough last night, and the only drawback as others have said, is the Draug fight.

Other than that, it's surprisingly fun. Maxing out your toxicity and going on a berserk weakness exploiting rampage is fun. I played on Hard and aside from a few fights I wasn't prepared for, was really fun. Using the Adrenaline boost then wiping off 75% of an opponent's health bar is amazingly fun, and by the end of the game I was causing instant kills when being poisoned.

Also, no matter what class you choose, always sink one talent point into the dodge skill. Whirl isn't necessary anymore with the 1.2 patch, but the extra dodge distance is a lifesaver.

Alchemy plus a level in daggers will solve the Draug fight. If you hurl several poisoned harpy claws (daggers) at the Draug, he won't even touch you.

I always felt that the potion duration SHOULD only be effected in combat and should stay uneffected during dialogs and non combat exploration until you meditate. This way drinking a potion and then getting yourself involved in 10 minutes conversation before the fight is not necessary a bad thing...

Alchemy path was fun and seemed very "Witchery" to me.

I use a potion mod, since some cut scenes are long and I don't want to feel compelled to skip through them. (It makes RP sense that timer keeps going during convo, since it's not like the world stops. It also may have been too tricky for them to start and stop the timer. In that case, the base duration just probably should have been a minimun 20 minutes.)

Modding will not over come the Draug problem, though. Whenever the game puts you in somebody other than Geralt, potions go away. Plenty of different tactics work there, though.

I have not gone back to check, but somebody pointed out elsewhere that the Alchemy Path passives don't seem to be working. I don't know if that applies to damage reduction. If that is so, it didn't make me feel underpowered. With Impregregnation, and the frequency of Berserker mode I had fun.