Dosage Calculation
Part of Herbal Medicine
Determining safe and effective herbal doses by age, weight, and condition when no pharmacist is available.
Why This Matters
“The dose makes the poison” — Paracelsus wrote this in the 16th century, and it remains the foundational principle of pharmacology. Every medicinal herb has a therapeutic window: too little accomplishes nothing; too much causes harm. Willow bark at the right dose relieves pain; at a very high dose, salicylates cause bleeding. Comfrey applied externally heals wounds; taken internally in large amounts for months, pyrrolizidine alkaloids damage the liver.
In a world without pharmacy labels or standardized pills, calculating dose from raw plant material requires judgment, basic math, and an understanding of how body size affects drug metabolism. Children metabolize compounds differently than adults. Elderly people with reduced kidney and liver function need lower doses. Pregnant women face special restrictions.
Getting dosage right separates effective herbal medicine from dangerous guessing. This knowledge belongs in every community’s medical repertoire alongside the herbs themselves.
Basic Dose Units for Herbs
Without measuring equipment, traditional medicine relied on standardized units that anyone could replicate:
| Unit | Approximate Volume | Use |
|---|---|---|
| Pinch | ~0.3 mL / ~0.1g dried | Spices, strong herbs |
| Teaspoon | ~5 mL | Most dried herb teas |
| Tablespoon | ~15 mL | Bulk dried herb |
| Cup | ~240 mL | Tea volume |
| Drop | ~0.05 mL | Tinctures, essential oils |
For teas: Standard adult dose is 1-2 teaspoons dried herb (or 1-2 tablespoons fresh) per cup of water, 2-3 cups per day.
For tinctures: Standard adult dose is 20-40 drops (1-2 mL) in a small amount of water, 2-3 times per day.
Adjusting for Age
The most critical adjustment is for children. Children are not simply small adults — their livers and kidneys process drugs differently, their blood volume is smaller, and their surface-area-to-weight ratio is higher.
Clark’s Rule (Weight-Based)
Most accurate method when you know the child’s weight:
Child dose = (Child weight in lbs divided by 150) multiplied by adult dose
Or in kilograms: Child dose = (Child weight in kg divided by 68) multiplied by adult dose
Example: Adult dose is 30 drops of tincture. Child weighs 44 lbs (20 kg).
- 44 divided by 150 = 0.29
- 0.29 multiplied by 30 drops = approximately 9 drops
Young’s Rule (Age-Based)
Use when weight is unknown:
Child dose = (Age in years divided by (Age + 12)) multiplied by adult dose
Example: Child is 6 years old. Adult dose is 2 teaspoons.
- 6 divided by (6 + 12) = 6 divided by 18 = 0.33
- 0.33 multiplied by 2 teaspoons = 0.67 teaspoons (approximately 2/3 teaspoon)
Simplified Age Guide
| Age | Fraction of Adult Dose |
|---|---|
| Under 1 year | Do not dose — use only very mild herbs (chamomile) at 1/10 |
| 1-2 years | 1/8 adult dose |
| 2-4 years | 1/6 adult dose |
| 4-7 years | 1/4 adult dose |
| 7-10 years | 1/3 adult dose |
| 10-14 years | 1/2 adult dose |
| 14-18 years | 2/3 to full adult dose |
| Adult (150 lbs / 68 kg) | Full dose |
| Elderly or frail | 1/2 to 3/4 adult dose |
Adjusting for Condition
Acute vs. Chronic Treatment
- Acute conditions (fever, acute infection, sudden pain): Higher doses, more frequent intervals. Example: yarrow for fever — full dose every 2 hours until sweating begins, then reduce to 3 times/day.
- Chronic conditions (ongoing digestive issues, joint pain, insomnia): Lower doses taken consistently over weeks or months. Example: valerian for chronic insomnia — standard dose once at bedtime, taken for 4-6 weeks.
Severity
Mild symptoms warrant mild treatment. Severe symptoms may justify the higher end of therapeutic range, but never exceed maximum dose guidelines.
Body Weight Extremes
A 90-lb elderly woman needs a smaller dose than a 220-lb man, even if both are adults. When treating someone significantly lighter or heavier than average, apply Clark’s rule using 150 lbs as the reference adult weight.
Special Populations
Pregnancy
Treat pregnancy as a special restriction category. Many herbs that are safe for adults are contraindicated in pregnancy because they stimulate uterine contractions or cross the placenta.
Avoid entirely during pregnancy: Pennyroyal, blue cohosh, black cohosh (in first trimester), tansy, wormwood, large amounts of sage or thyme, juniper berries, large doses of any bitter herb.
Generally safe in moderate culinary amounts: Ginger (for morning sickness), chamomile, peppermint, fennel.
When in doubt, do not administer any herb beyond food quantities to a pregnant woman.
Breastfeeding
Compounds in herbs pass into breast milk. Doses should be conservative. Chamomile, fennel, and ginger are generally well tolerated. Avoid herbs with alkaloids (comfrey, borage), stimulants (large amounts of caffeine-containing plants), and anthraquinone laxatives (senna, cascara).
Kidney or Liver Impairment
Herbs are metabolized by the liver and excreted by the kidneys — just like pharmaceutical drugs. If someone has chronic liver disease (jaundice, alcoholism, hepatitis) or kidney disease (reduced urination, swelling), reduce herbal doses by half and avoid herbs with known hepatotoxicity (comfrey, kava, large amounts of valerian).
Calculating Tea Concentration
Standard tea concentrations vary depending on intended use:
| Strength | Herb per Cup | Use |
|---|---|---|
| Mild | 1/2 teaspoon dried | Preventive, daily tonic |
| Standard | 1 teaspoon dried | Most therapeutic teas |
| Strong | 2 teaspoons dried | Acute symptoms |
| Decoction | 1 tablespoon per cup, simmered | Roots, bark, tough material |
For a day’s supply of medicine, prepare a full pot (4 cups) using 4 times the per-cup measurement, then divide through the day.
Monitoring Response and Adjusting
Start low, go slow — especially with unfamiliar herbs or sensitive patients. Begin at half the calculated dose for the first 2-3 days. If well tolerated and insufficient effect, increase to full dose.
Signs of correct dose: Symptom improvement within expected timeframe (e.g., fever reduction within 1-2 hours for diaphoretic herbs, pain relief within 30-60 minutes for analgesics).
Signs of too much: Nausea, headache, skin rash, unusual fatigue, increased heart rate, excessive sweating beyond therapeutic effect. If these appear, stop the herb and give water.
Signs of too little: No response after 3-4 days of consistent use at labeled dose — either increase dose, change preparation method, or reconsider the herb choice.
The 10% Rule
When using a new-to-you plant for the first time, give 10% of the calculated dose as a test dose. Wait 30 minutes. Watch for any allergic reaction (rash, swelling, difficulty breathing). If none, proceed with full dose. This catches rare but dangerous sensitivities before a full therapeutic dose causes serious harm.