This issue came to the fore again last session when Tilia was faced with a daunting magical challenge and she ate some vile smelling fungal growth and a wing of the staring moth to gain bonuses they would give to her arcana check (both items collected during the course of the campaign).
This is still tentative but I have a rough mechanic. For each pair of herbs ingested, roll 1d20. A result of 1 indicates a contraindication and a result of 20 indicates an unexpected beneficial effect. When more than two herbs are ingested at once, roll for each individual pair and also once for the entire unique combination.
I have a very rough start on contraindication and unexpected beneficial effect tables (these definitely need to be expanded to at least d20 tables - leave suggestions in the comments?): EDIT: Mike, player in my 4e campaign and sometimes poster on this blog, sent me an email with a bunch of great suggestions for this that I will be incorporating into these tables soon.
Contraindication Table: 1d12
Remember to tell your doctor about every magical herb you are currently taking when discussing a new prescription!
2. This combination of herbs inhibits the body's ability to process alcohol for 1d4 days. Not only will alcoholic beverages be dramatically more potent, the drunk will last for the entire duration and a number of drinks = 1/3 Constitution could result in death by alcohol poisoning.
3. Combining these herbs interferes with the efficacy of magical healing. -2 per die on all magical healing for 1d4 days.
4. These herbs just don't work very well together. 1 (1-5 on d6) or both (6 on d6) of the herbs work at 1/2 normal effect (whatever that is) when combined.
5. Severe light sensitivity results for 1d6 days, blinded in full daylight and -2 to hit in bright to moderately lit rooms. If a 6 is rolled for duration, the light sensitivity continues for another 1d6 days. If a 6 is rolled on 3 successive checks, the effect is permanent.
6. A numbing of all physical sensations occurs, causing a -4 penalty to Dexterity and the inability to feel pain for 2d4 hours.
7. Do not combine these herbs when pregnant. This could be as simple as a chance of miscarriage or as crazy as a chance of magical birth defects. Alternatively this could pollute the semen of male characters for 3d10 days, either causing sterility or hyperfertility for the character and magical birth defects for any children conceived during that period.
8. One of the herbs (randomly determined) does not function at all while the other functions at 150% normal efficacy.
9. A severe drowsiness results, a fast and unavoidable sleep descending in 3d6 rounds and lasting 2d30 hours. Although pain or sudden loud noise can wake the character, sleep will return in 3d6 rounds until the original duration has passed.
10. A slow process of petrification starts. This initially manifests as +2 to AC and -4 to REF. Over the next 2d6 days, roll a save at -1 cumulative per day the petrification has progressed. Each failed save results in an additional +2 to AC and -4 to REF. Each successful save reverses one days effects (if the first save is made the petrification ceases). When REF reaches 0, the body is turned completely into stone and can only be cured through magical means.
11. Dehydration/nausea results (Thanks Mike!). Rolling a 1 on any attack/skill check dazes you for 1d4 rounds. Rolling a 1 twice without stopping to rest and rehydrate causes dehydration to the point where you fall unconscious for 1d4 rounds and can not be roused by normal means. Falling unconscious twice in this manner causes you to fall into a coma for 1d4+1 days, where you must seek proper medical treatment via someone trained in the Heal skill. Failing to do so within 2 days means you make a Death saving throw, if successful you stabilize and remain in the coma for an additional 1d4+1 days (added on to the first roll) You may also suffer permanent Ability damage. After 2 more days without medical treatment you automatically perish. Receiving proper medical treatment within 2 days prevents the Death saving throw, but you will remain in your coma for the full duration of your total days rolled. (Min 4 - Max 10)
12. Combining these herbs can be instantly fatal. Saving throw or die in 4e (or savings throw vs. Death).
11. Dehydration/nausea results (Thanks Mike!). Rolling a 1 on any attack/skill check dazes you for 1d4 rounds. Rolling a 1 twice without stopping to rest and rehydrate causes dehydration to the point where you fall unconscious for 1d4 rounds and can not be roused by normal means. Falling unconscious twice in this manner causes you to fall into a coma for 1d4+1 days, where you must seek proper medical treatment via someone trained in the Heal skill. Failing to do so within 2 days means you make a Death saving throw, if successful you stabilize and remain in the coma for an additional 1d4+1 days (added on to the first roll) You may also suffer permanent Ability damage. After 2 more days without medical treatment you automatically perish. Receiving proper medical treatment within 2 days prevents the Death saving throw, but you will remain in your coma for the full duration of your total days rolled. (Min 4 - Max 10)
12. Combining these herbs can be instantly fatal. Saving throw or die in 4e (or savings throw vs. Death).
Unexpected Beneficial Side Effect Table: 1d10
1. A random stat is increased by 1d4 for 1d6x10 minutes.
2. These herbs work great together. 1 (1-5 on d6) or both (6 on d6) work at 150% normal efficacy.
3. This combination reduces fatigue and hunger. No food or sleep needed for 24 hours. No side effects.
4. This combination of herbs restores 3d6 HP or grants 3d6 temporary HP that disappear if not used within 2d8 hours.
5. Lucky combination: +1 to all rolls for 24 hours
6. A metabolite of this combination is sweated out and is irresistible to a random species (25% chance of being the same species as the ingester), acting as a love potion upon first exposure and a charm person/monster for subsequent suggestions made by the ingester, for 1d4+1 hours. Even species not affected will find the body odor of the ingester very appealing for the duration.
7. Magically toughens the ingester, +2 to AC for 3d6 hours.
8. Grants telepathy to 50'. This requires concentration and can only be used on one subject at a time. Does not require line of site. All living minds within 50' can be detected and contacted with concentrated effort.
9. Moment of Zen. Exact effect of this is up to the player and DM. Suggestions of a mechanical nature would include temporary bonuses to Wisdom, initiative, diplomacy, reaction checks, perception; suggestions of a more flavorful nature would include giving the DM a blank card in the form a character "revelation" (the DM gets to either reveal a secret or make up some shit about the campaign world) or some kind of breakthrough on a personality or character flaw. This effect can never be intentionally recreated, only occurring as a random side effect of different herbal mixtures.
10. Restores youthful vigor - lose 1d4 years of age. Each time this combination of herbs restores youth there is a 1% cumulative chance of developing an irrational and overriding fear of aging.
Standardizing Doses and Duplicating Results
Unless very exacting precautions were taken to both accurately measure the potency of each herbal specimen and the exact amount consumed, combining the same types of herbs will not necessarily yield the same result. I think a good start would be to give a 25-75% chance of recreating a contraindication or beneficial side effect, with the care taken to measure dosage determining the exact chance. This assumes the same batch of herbs was used - I would lower these chances if this was not the case. The 25% chance of failure even given the same herbs and very careful measurement of dosage means that there was some other factor present the first time that is missing. Perhaps a fly flew in the preparation, or it was a hotter or colder day and the reaction depends on a certain temperature, or it was mixed in a copper bowl the first time and a wooden bowl the second, etc.
Addiction
Yes of course there should be addiction rules but I really haven't nailed that one down yet. I am still kicking around what sort of mechanic I want to use for addiction. I don't want it to be a record keeping game of tracking doses used per period of time, I know that much.