This recommendation for The Guide to Herbs for RPGs cites
my own experience using it for the purpose in the question.
I've been using Shaun Hately's venerable The Guide to Herbs for RPGs, first publicly published in 1997 and now in its fifth edition. Since it can be freely redistributed, it can be found in several places, including in HTML as The Illustrated Guide to Herbs for RPGs, 5th Edition, as well as in its original PDF format hosted on Google Drive.
The virtue of this herbal is that the medicinal effects it lists are very focused on what is relevant to the kinds of circumstances that come up in fantasy RPGs: a lot of healing herbs, herbs that can be used to defeat poison, herbs that can be used as legendary MacGuffins for "cure the dying king" quests, and so on. Herbs list the season, climate (temperate, tropical, etc.), and environment (forest, coastal, etc.) in which they can be found. These categories make provisions for fantasy-centric herbs too — for example, some fantastical herbs list "volcano", "underworld", or "corpses" for where they can be found. Its RPG-ness can also be seen in how it uses dice to determine various things, such as how long an effect might last or its rules for how long dried herbs maintain potency. It also intersperses advice to GMs on customising the herbal to their campaigns' needs.
The herbal is not system-neutral, but the effects of the herbs often are. Those that do involve mechanics use simple terms derived from D&D that can be converted to or used as-is in other RPGs easily enough.
Furthermore, it provides rules for locating, preparing, preserving, and buying and selling the herbs in terms that are immediately useful to a GM or player trying to use it during play. These rules are also loosely based on D&D, but they are general enough and a-mechanical enough that they can be converted to most any other RPG fairly easily, usually just by swapping out an equivalent skill. (I'm using the Guide in RuneQuest 6, for example.)
The appendices also include lists that collate the herbs by their season, climate, environment, and whether they are real or fantasy herbs, so that you can prune the herbal to the needs of your specific campaign. The last list means that it's easy to adjust the herbal for mundane games where fantastical herbs (like the “cephalophage” mushroom) would be inappropriate.
The only thing I have found it lacks for game utility is a way of randomly selecting herbs, which would be useful for generating herbalist shops' inventories or the medicinal contents of a specific hex on a map, but this is a minor complaint.
Two example entries from the herbal
Here are a pair of herbs from the Guide, to illustrate where it is and isn't system-agnostic and how much conversion may be needed. (Note: the name in parenthesis is the contributor, not the “Latin” name; the percentage in ‘Available’ is the daily chance to find the herb when actively searching for it, while ‘Uses’ is how much that search yields; price slash shows raw vs. prepared.)
Horseweed (Maya Kniese)
Available: Spring, Summer, Autumn 10%
Climatic Zone: Temperate
Cost: 7 gp / 7 gp
Ability Check: Intelligence −2
This 25 cm-high plant has many broad, double-sawed leaves growing in a rosette. It grows large, yellow composite flowers, which will quickly grow hairy seeds that will fly far on the wind. The plant often has flowers and seed simultaneously. If a generous amount of the leaves is fed to a horse or similar creature, it will be able to travel all day without tiring, even if moving at a gallop.
Most of this can be used as-written in any game: availability, climate, locale, preparation, uses, and effect are all system-neutral. Cost is almost system-neutral, since most FRGPs use the gold standard. The main mechanical consideration for using this with some RPGs is adjusting the AD&D-style Ability Check that determines whether it has been successfully used and therefore has its effect. For example:
- RQ6: a Lore (Herbalism) skill roll would be substituted for the Int check, and it would be made at Standard difficulty grade (since −10% is too little to bother bumping it to Hard). Cost would be the same number but in silver. Effect works as-written.
- D&D 5e: A DC 15 (Moderate) Intelligence check + Proficiency if the PC has an herbalism-related skill. Effect works as-written.
Marsh Mallow (Shaun Hately)
Available: Always 20%
Climatic Zone: Temperate
Preparation: 1 day
Cost: 5 gp / 8 gp
Ability Check: Intelligence −7
This plant has a green fleshy stem and broad egg shaped leaves, both of which are covered with downy hairs. It has five petalled, pale pink flowers which sit at the base of the leaves. The root must be boiled and the resultant distillation applied to burns. If used successfully the herb will double the rate of healing for the burn.
Here the effect may need conversion as well. For example:
- RQ6: Use Lore (Herbalism) at Formidable difficulty grade. Read the cost in silver instead of gold. Effect works as-written.
- D&D 5e: DC 20 (Hard) Intelligence check + Proficiency if they have an herbalism-related skill. Make the effect "If successful and you've taken fire damage recently, the herb lets you roll and sum two dice per HD spent to heal during a short rest."
Such conversions aren't exact, but the effects are made up in the first place, so a near-enough conversion has the same precision as the originals anyway. Converting is simple enough it can be done on the fly, or by copying the text into a word processor and making the adjustments ahead of time.