Any substance is a medicine at one amount and a poison at a larger one, and the ratio between the harmful amount and the useful amount, the therapeutic index, tells you how much error a medicine tolerates. A medicine with an index of 100 forgives a ten-fold mistake; one with an index of 3 does not forgive a four-fold one. A home-made preparation of unknown strength can swing by several times between batches, so for narrow-index substances no dosing rule is safe.
Part of Organic Chemistry
The relationship between dose, effect and harm, and why unknown preparation strength defeats every dosing rule.
The Margin Is What You Are Really Buying
A dose-response curve has two thresholds: the amount that starts to help and the amount that starts to harm. The distance between them is the margin. This is the content of the carrier’s subtitle, “the dose makes the poison”, and of its warning that every alkaloid is toxic at high enough doses.
Treat the margin as a ratio. Define the therapeutic index as the harmful amount divided by the useful amount. A high index means the substance is forgiving of measurement error, a low one means the same error can poison.
What Makes The Effective Margin Smaller Than The Textbook One
The textbook index is measured for a fixed, known material. At home several things eat into it.
- Unknown strength. The preparation may be weaker or stronger than the last one. See Standardization.
- Body size and age. Children, the old, the underweight and those with liver or kidney disease clear drugs more slowly. The corpus’s dose-scaling pages are Dosage Calculation and Dosage Calculation (pharmacy); the reference doses for one class of drug are in Dosage Guidelines.
- Accumulation. Repeating a dose before the last is cleared stacks the amounts.
- Interaction. A tincture carries ethanol, and other medicines or foods can change absorption.
- Route and form. The same material acts differently as a tea, a tincture or a concentrate (Dosage Forms).
The Corpus Carries Two Dose-Scaling Schemes; They Are Not Reconciled Here
The two dosage-calculation pages named above scale a dose differently: one leads with Clark’s rule (child weight against a 68 kg adult) and Young’s rule, and the other with a milligram-per-kilogram figure and a note that Young’s rule is less precise. Neither alters the other’s numbers here. The pharmacy page’s own instruction applies: when two methods give different answers, use the lower estimate and monitor the patient. These rules also assume the adult dose is a real, tested one. They give no protection when the strength of the preparation is unknown.
Method: A Safe Habit For Any Medicine
Step 1: Ask for the margin first. Is the preparation wide-margin (a mild tea) or narrow (a concentrate, a cardiac or opioid compound)? If narrow and untested, stop.
Step 2: Fix the preparation so each dose is made the same way.
Step 3: Start at the low end and give nothing further until the first dose has had time to act. The carrier’s own advice for alkaloids is to start with a small fraction of the estimated dose and increase gradually, keeping detailed records.
Step 4: Record who, what, when, how much and the result.
Worked Example: What A Six-Fold Spread Does To A Narrow Margin
Use hypothetical units, not any real drug. A preparation is useful at 10 units and harmful at 30 units, so its therapeutic index is 30 ÷ 10 = 3. A wide-margin preparation is useful at 10 units and harmful at 1,000, an index of 1,000 ÷ 10 = 100.
Now suppose batch strength varies six-fold between the weakest and strongest batch (the same size of spread worked in Alkaloid Extraction). A volume that delivers 10 units from the weakest batch delivers 10 × 6 = 60 units from the strongest.
- Narrow margin: 60 exceeds the harmful 30. The same volume is a medicine on one day and a poison on another.
- Wide margin: 60 is well under the harmful 1,000. The error is harmless.
That is the case for preferring wide-margin preparations and for refusing to dose narrow ones without measurement.
Common Mistakes
- Doubling a dose that “did nothing”. The first dose may be slow to act or the batch weak; the second may be on top of a delayed first.
- Judging safety by a person’s size alone. Strength of the preparation matters at least as much.
- Giving a child’s dose by guesswork. Use the written scaling rules and the lower estimate.
Safety
A dose given to a person is not something to improvise
For narrow-margin plant compounds this corpus gives no extraction procedure and no dose. If a substance has a published, tested dose, follow it exactly, record it, and start low. Keep preparations labelled, dated and locked up; poisoning in a household is usually a child or a mislabelled bottle, not an informed adult.
What’s Next
- Learn next: Standardization — how to make each batch resemble the last
- Combine with: Natural Pharmaceuticals — which compounds are worth the effort