I'm basically using the rules in GURPS Compendium II on the finding and preparation of herbs. The skill Herbalism is Hard, and covers both collection, preparation, and administration of medicinal herbs. Note that Naturalist skill will also suffice to collect herbs, Alchemy skill allows the preparation of herbs, and Physician skill allows the administration of herbs.
The herb listings below show how frequent the herb is in nature (and usually where one might expect to find it), how much each "dose" costs to buy in ready to use form, what the finished product looks like, and what its effects are.
Alanal:
Common, 6d per dose, grows in heathland.
A yellow, bitter crystalline powder, taken orally.
Effect: Hallucinogen, takes effect in 4d6 minutes and lasts for 1d6+6
hours. Psychologically addictive, may cause brain damage over time
(worth -15 pts as a disadvantage).
Arva:
Rare, 120d per dose.
A thin, lavender decoction, sweet to the taste.
Effect: "Strength Potion"; beginning after 3d6 minutes, user's ST is
increased by 2. This lasts for HT*6 minutes; afterwards, take
Critical fatigue and make a HT roll to avoid muscle strain (a Light
injury, Medium on a critical failure).
Berelik:
Average, 30d per dose, grows in shady forests.
A greenish-yellow oily paste, it is applied to open wounds as a
poultice.
Effect: Anaesthetic (effectively gives the user High Pain Threshold,
or halves pain penalties again if the user already has that advantage)
for 1d3+1 hours, after which numbness and lethargy set in. The
anaesthetic effect still lasts for 1d6+4 hours after that, but now
accompanied by Serious fatigue and an extra -1 to DX. Also, it is
mildly antibiotic and gives a +2 bonus to avoid infection from any
wound it has been applied to. Each dose suffices for one wound.
Doshenkana:
Common, 15d per dose, grows in most forests.
A dull-green, bitter powder, half an ounce taken orally.
Effect: Nausea slowly builds over 3 hours, accompanied by increasing
breathlessness and gradually blurred vision. During this time, the
poison is gradually causing a Critical internal wound; if effective
treatment (vomiting will work in the first 30 minutes, Yulpris or
magic are the only cures after that) is not given, the victim must
make a HT-2 roll or die. Survivors must roll vs. HT+1 or suffer
permanent damage to their vision -- take a penalty on all vision
rolls equal to twice the margin the last HT roll was missed by (on
a miss by 5 or more, the victim goes blind).
Elprequir:
Common, 12d per dose, grows in pastures.
Dried and crushed, ground into a fine blue powder, taken orally.
Effect: A common aphrodisiac, often used clandestinely in food.
Whether it has any real effect is an open question, but a lot of
Hârnians swear by it.
Fanosel:
Average, 30d per dose, grows on the floor of mixed and needleleaf
forests.
Mashed, left to dry and crystallize; then drunk as tea.
Effect: Causes euphoria and reduced inhibitions; beginning after
5-15 minutes and lasting for 2d3 hours, the user becomes giddy,
Overconfident, and Impulsive; usually Lecherous as well. Afterwards,
the user becomes depressed and irritable (Bad Temper) for 1d3 hours.
Psychologically addictive, with a -5 on Will rolls to withdraw (worth
-15 points as an Addiction).
Fletharane:
Average, 20d per dose, grows in light forest.
Normally used as a ropey plant fibre, which is burned and the smoke is
inhaled.
Effect: Mild euphoriac and hallucinogen, with effects lasting about
an hour. Aftereffects include lethargy and hunger. Generally legal,
even though unwise use (excessive doses burned in enclosed places)
may cause partial memory losses.
Gaethipa:
Average, 60d per dose.
A white, sticky gum. Blood poison, usually applied to weapons (most
often arrowheads).
Effect: 5d6 seconds after introduction to bloodstream, the victim's
central nervous system begins to sputter and malfunction, causing
symptoms similar to grand mal epileptic seizure. Over the
next minute, three HT rolls must be made (one every 20 seconds); if
any one of them fails, the victim suffers immediate Cardiac
Arrest; if all are made, the victim only goes Unconscious. After 1d6
hours, the victim must make another HT roll or die anyway. Survivors
must make a HT roll or suffer permanent damage -- for each point by
which the HT roll missed, take -10 pts of disadvantages or reduced
attributes (DX or IQ loss from general nerve damage, or partial or
total loss of vision or hearing, are most common). This very
dangerous substance has no known antidote or cure, except
for magic or miracles; it is not possible to build up an immunity to
it. It is seriously illegal in all civilized countries, although some
"barbarian" tribes (as well as some assassins) use it.
Habsulara:
Common, 6d per dose, a blue flower which grows in pastures.
Dried and powdered, inhaled or allowed to melt inside the mouth.
Effect: Stimulates the brain (+1 IQ) for 1d3 hours; may cause nausea
(-1 to everything physical) on a failed HT roll (a critical failure
will give -3 to everything instead). As the effect wears
off, expect to lose 10-30 minutes of work time to headache and
coughing. Psychologically addictive, with -5 to Withdrawal rolls;
prolonged use may cause brain damage and stroke (worth -10 pts as
a disadvantage). Illegal in several civilized nations.
Hreyochor:
Average, 30 d per dose.
Dried and powdered, taken orally (usually in food or drink).
An effective fertility drug for women; adds +2 to monthly chance of getting
pregnant (and increases chance of bearing twins to a whopping 25%).
It is in fairly common use, most often administered by Peonian
priestesses to women who still won't conceive after intensive prayers
have been brought to bear.
Joldraiven:
Rare, 150d per dose.
A dried, 2' long plant with red leaves, given to horses as food.
Effect: On horses, causes temporary increase in speed (+1
Move) and endurance (Very Fit) for 1d6 hours, but then leads to
Critical fatigue (and a HT roll to avoid muscle damage -- a critical
failure here kills the poor horse). Frequent use may cause permanent
muscle damage. On humans, eating the plant would cause
rapid nausea and vomiting (its taste is vile, so you probably couldn't
eat very much of it either).
Kargele:
Rare, 150d per dose, grows on mountainsides above the tree limit.
Powdered and dried, it is formed into a small cake, which is
eaten.
Effect: Quickly leads to drowsiness and sleep which lasts for 6d6+14
hours; upon waking, patient has a voracious appetite. Gives a +2
bonus to HT for healing purposes only from the time the
dose is taken and for the next 1d6+3 days (during which time the
patient will require 50% more food than normal and suffer from
constant Light fatigue).
Lavaryctia:
Average, 30d per dose.
A yellow, bitter concotion, which is taken orally.
Effect: Soporific; building drowsiness leads to sleep within 1d6*5
minutes. Unwilling users may try to stay awake by making Will rolls
at the same frequency as if they'd been awake for 16 hours past their
normal bedtime, and will have -3 on IQ, DX, and skills. The effect
wears off after 1d6+6 hours. An increased dose will require a HT roll
to avoid coma, at -1 per extra dose beyond the second; a critical
failure here causes Cardiac Arrest.
Leortevald:
Common, 6d per dose, grows in deep forests everywhere.
Dried and powdered, mixed in drink or food.
Effect: Basically the same as for hemlock (see Compendium II).
Olrui:
Average, 26d per dose, found as a fungal parasite on dying trees.
Mashed, then left to dry and crystallize; dissolved in hot water and
drunk as tea.
Effect: Stimulant. Prevents fatigue from lack of sleep for 18+3d6
hours, and effectively makes the user Very Fit for this duration.
After it expires, the user suffers Critical fatigue, and will need
12 hours of sleep ASAP.
Perigwar:
Average, 30d per dose, a spring flower in pastures.
A violet, fragrant decoction, often mixed with whale oil for
consistency. Applied as perfume.
Effect: Aids sexual arousal in both genders (say, +1 to Sex Appeal)
for 1d3+1 hours.
Quessel:
Average, 30d per dose, spring pollen from a flower.
Dried pollen is a fine yellow powder; often mixed with a nut paste or
something to keep it from getting lost.
Effect: Mild antibiotic, giving +1d6 to HT (roll once per disease)
versus diseases and infections if taken when the first symptoms show
up, as well as giving half that bonus to aging rolls for the next
year (if multiple doses have been used, you only get the
best bonus).
Rasakile:
Common, 6d per dose.
A blue-green decoction, usually heated before drinking.
Effect: Contraceptive for females, one dose must be taken each day for
continual effect. Subtracts -3 from chance of getting pregnant;
prolonged use may cause permanent partial or total sterility (make a
HT roll after each 360 days of use; on a failure, take a permanent -1
to chance of pregnancy).
Stiltrassa:
Average, 60d per dose.
A sweet, white, viscous decoction, taken orally.
Effect: Powerful emetic and laxative; empties digestive system within
10 minutes and causes Serious fatigue. On an overdose, it may cause
internal hemorrhaging. Generally used as an antidote for digestive
poisons.
Tasparth:
Common, 3d per dose, grows everywhere.
After preparation, it's a reddish-brown powder, which is burned and
the smoke inhaled.
Effects: Use the same mechanics as for Peyote (in Compendium II), but
it's easy to find, so reduce point cost for addiction to -10 pts.
Teranya:
Average, 30d per dose.
Blue-green, bitter decoction, dripped into each eye.
Effect: Stimulates recovery from blindness caused by disease or
cataracts; make a roll vs. HT-1, and on a success you'll regain
your vision. Further attempts rarely succeed if the first one
failed, and it's no good vs. physical damage to the eyes.
Tirageyth:
Average, 15d per dose, woodland flower.
Leaves are dried, and chewed.
Effect: Euphoria for 5d6 minutes, leading to drowsiness (Serious
fatigue). Overdoses may cause coma, prolonged use causes brain
damage (addiction disadvantage: -15 pts).
Wylorafina:
Average, 30d per dose.
A sour, red decoction, taken orally.
Effect: Causes heart fibrillation after about an hour, and (on a
failed HT roll) Cardiac Arrest. Survivors risk permanent loss of 1
level of HT if another HT roll is failed (losing 3 levels of HT on
a critical failure). This poison is not very commonly used since
it's not very reliable. While it also gives a bit of a heady rush,
it's not much good as a recreational drug since most users don't
survive all that many doses.
Yulpris:
Very Rare, 600d per dose, only grows in boggy soil on Setha Heath.
The plant is mashed and dried to a green crystallized dust, then mixed
with hot water and drunk as a tea.
Effect: Greatly stimulates the body's own immune defenses, giving a
blanket +5 to HT vs. all diseases and most poisons, for a
month.