I'm of the mind that Alchemy and the substances made from it really aren't that useful. They'd be great early on, but most Crafting DC's are too high, or the substances are too expensive, or time consuming to make, often all of the above. At higher levels you have better ways to get buffs, so Alchemy gets left at the way side. If you keep scaling the difficulty to obtain the substances, simply making better substances doesn't alleviate the problem at all. To this end I took a page from Fallout and decided that there could be very good alchemical compounds, but you'd run the risk of addiction by using them. The system below is to deal with the chances of addiction, the consequences of it, how to fight it, and the effects of withdrawal. I'm attempting to hit a nice middle ground between useful and realistic here, so some of this might not be perfectly representative. If you want to see some of the compounds that I'm adding with this, check the bottom of the post.

Chances of Addiction

Most potent alchemical compounds carry with them the risk of addiction when used. These compounds are split into three tiers, Weakly Addictive, Addictive, and Highly Addictive. If a particular compound is used more than once within a certain time span (1 day, 1 week, 1 month for successive tiers Weak -> High) the chances of becoming addicted are increased by +1 for each tier level (+1/+2/+3) each time, stacking up to three times (+3/+6/+9). A compound which is Weakly Addictive carries a DC 12 Fortitude save to avoid becoming addicted. An Addictive compound has DC 15, and a Highly Addictive compound has DC 18.

Consequences of Addiction

Note: The rules below assume attempting to quit cold turkey. Attempting to ween yourself off, or other methods/practices to make it easier will temper the difficulty. The amount is at the DM's discretion, as per usual.

If a character becomes addicted they can attempt to fight off the addiction with successive Will saves of the same DC as the Fortitude saves to avoid addiction. Weakly Addictive compounds use 3 Will saves, Addictive compounds use 4, and Highly Addictive compounds use 5. These saves are made once a month. If a save is made the DC of the next check is lowered by the characterís Wisdom mod, minimum 1. If a failure occurs at any point the DCís are reset and the process start over again. If a character manages to fight off addiction to a substance they will forever suffer a -2 penalty against avoiding the addiction again. If they need to fight off addiction to the same substance they also suffer a -2 penalty.

Additionally, if a character is attempting to break an addiction, they roll Will saves if presented with a good excuse or an open opportunity to take the compound they are addicted too. Both instances carry a -2 penalty on the save and they stack. If failure occurs the DCís for breaking the addiction are reset. If a critical failure is rolled in opposition to addiction at any point the DCís are increased by 1 permanently.

Withdrawal and its Effects

Withdrawal symptoms occur shortly after an addict is no longer under the effects of their compound. Withdrawal symptoms vary from person to person and the circumstances can influence what symptoms manifest. This means the following tables are guidelines only and subject to DM fiddling. Withdrawal symptoms will set in rapidly if the addict canít get ahold of their compound, at a rate of 1 per day. Weakly addictive compounds will have 2 symptoms, addictive will have 3, and highly addictive will have 4. Roll 1d20 to determine which symptoms manifest. If an odd number of symptoms are needed select a physical symptom if base Constitution<Wisdom, and an emotional symptom if Constitution>Wisdom. Symptoms will persist for two weeks for a Weakly Addictive substance, at which point one symptom will disappear and the remaining symptom will be re-rolled. Addictive compounds are similar, but the symptoms stick around for three weeks before they start to disappear. There is also a chance that an Extreme Withdrawal Symptom can appear. Highly Addictive compounds behave in the same manner, with full symptoms lasting five weeks, and an increased chance of Extreme Withdrawal Symptoms.

(For those curious, all below are actual potential withdrawal symptoms. Don't do drugs kids!)

Physical Withdrawal Symptoms

Weak Addictive Highly Symptom Effect
1-2 1-2 1-3 Diarrhea Treated as though Sickened frequently. Have fun RPing this.
3-5 3-5 4-5 Palpitations Treated as though Fatigued frequently.
6-8 6-8 6-8 Muscle Tension -1 Str and Con due to prolonged tension.
9-11 9-10 9-10 Tightness of Chest Applies debuff equivalent to Endurance feat.
12-14 12-13 11-13 Difficulty Breathing -2 Con to simulate lack of oxygen.
15-17 14-16 14-15 Tremors -2 Dex due to loss of fine motor skills.
18-20 17-19 16-18 Nausea/Vomiting -2 Str due to lack of nutrients and stress on the body.
n/a 20 19-20 Extreme Symptom Roll one from the Extreme Symptoms Table

Mental Withdrawal Symptoms

Weak Addictive Highly Symptom Effect
1-2 1-2 1-2 Anxiety -2 Int; inability to discern between rational and irrational thoughts.
3-5 3-5 3-4 Restlessness -2 Wis; inability to think clearly. Does not apply yo saves against addiction.
6-7 6-7 5-7 Irritability You are one attitude worse towards everyone (Player's Handbook, pg 72)
8-10 8-10 8-9 Insomnia -2 Spot/Search, and cannot sleep.
11-12 11-12 10-11 Headaches +5% Spell Failure; applies to Divine as well.
13-15 13-14 12-13 Poor Concentration -5 Concentration
16-17 15-17 14-16 Depression -2 Cha; you aren't as 'fun' to be around.
18-20 18-19 17-18 Social Isolation -2 Bluff/Diplomacy/Gather Information
n/a 20 19-20 Extreme Symptom Roll one from the Extreme Symptoms Table

Extreme Withdrawal Symptoms

Rollaa Symptom Effect
1-4 Grand Mal Seizure Character becomes Catatonic. Convulses for 10+1d10 rounds, taking 1d4 damage each round. -2 Dex and Str, and Fatigued for the rest of the day.
5-8 Heart Attack Have a DC 30 Heal check within 20 rounds to avoid. If check is failed by 5 or more Heart Attack occurs. If it happens roll Fortitude. 20 and over nothing happens. 15-19 los 2 Con permanently. <15 character dies.
9-12 Stroke Roll 1d3. 1 gives -1d3 Int, Wis, and Cha; 2 gives -1d3 Str, Dex, and Con; 3 gives -1d12 Balance, Concentration, as well as Nausea/Vomiting. Fortitude save using Int instead of Con with DC of 20 lets you recover 1 ability point/4 days or eliminate the Balance, Concentration, and Nausea in the same manner. One attempt a week, three attempts total. Status recovered from is random. For every failure one debuff will remain permanently
13-16 Hallucinations Treated as though under the effects of Major Image frequently. Hallucinations determined by the DM.
17-20 Delirium Tremens Roll two additional symptoms, each, from the standard tables. Re-roll if symptom already exists, or is Extreme.

I am aware Delirium Tremens can only be brought on via alcohol normally and that it carries different symptoms than the ones on the two standard tables, but again, didn't want to go completely off the deep end here. Besides, not like it's one of the worse ones anyways.

Should also point out that although I'm fairly confident in this system I am less so about the compounds below. As such, they likely have balance issues on both sides of the spectrum.

This is Your Brain on Drugs!

Bolt: Addictive. Gain the effects of Haste for 10 minutes.

Hydra: Highly Addictive. Gain Fast Healing 10+1/12 max hp for 10 rounds. Can reattach severed limbs. Natural healing reduced by 50% for one week afterwards.

Rampage: Weakly Addictive. Gain Rage and DR 3/-.

Hulk: Highly Addictive. Gives Bull's Strength, Bear's Endurance, and +4 on Reflex saves for 5 rounds.

Ascension: Highly Addictive. Grants bonus on all rolls equal to one half character level, +2 Dex, Int, and Cha for 10 rounds. May override half character level bonus with full character level up to three times per dose. -2 to all ability scores for one hour afterwards.

Stable: Weakly Addictive. Gain Calm Emotions, +2 Concentration, +1 vs Morale Effects.

Leth: Weakly Addictive. Gain Calm Emotions, recover 1d8 hp, and an additional 2d8 non-lethal hp.

If you have any ideas for more I'm all ears. And as the title would suggest, any feedback is much appreciated.