Dosing equivalence: How to compare drug strengths safely

Dosing equivalence means matching how much active drug someone gets when formulations, salts, or units differ. It's the practical trick behind switching brands, changing from tablets to inhalers, or comparing doses between countries. Getting it wrong can mean no effect or extra side effects, so a little care goes a long way.

Start by finding the active ingredient and its units. Labels can show mg, µg, IU, or percentage. Remember: 1 mg = 1000 µg. For biologics and vitamins you may see IU — that unit depends on the substance, so don't mix IU with mg without checking. If you only compare brand names you can miss big differences; focus on the active ingredient.

Salt forms and molecular weight matter. Many drugs are sold as hydrochloride, acetate, or other salts. The label weight includes the salt, so 100 mg of a salt may contain less of the base drug. Pharmacists or official drug monographs give exact conversion factors. When in doubt, ask your pharmacist rather than guessing.

Common conversion situations

Switching inhalers: Metered-dose inhalers list micrograms per puff. Two inhalers with different propellants or dose counters may still deliver similar micrograms, but technique and device resistance change how much reaches the lungs. Match mcg per puff and prescribed puffs per dose, and practice with a spacer if needed.

Immediate vs extended-release: An ER tablet gives drug slowly. You can't safely split an ER pill or replace it with the same mg of an IR tablet and assume equal effect. Timing, peak levels, and side effects will change. Talk to your clinician before switching formulations.

Topical, oral, and IV differences: The same mg taken by mouth isn't the same as IV. Oral bioavailability can be low for some drugs. For eye drops or topical creams the amount absorbed systemically is often tiny. Use the right conversion for the route — pharmacists can help you translate between them.

Simple checks before switching doses

1. Read the label: active ingredient, amount, salt form, and units.

2. Convert units: mg vs µg — use a calculator for accuracy.

3. Match formulation: don't swap ER for IR without advice.

4. Consider route: inhaled, oral, IV, topical — effects vary.

5. Ask a pharmacist: they have equianalgesic and conversion charts and can spot risky swaps.

Use reputable tools and official monographs for complex drugs. If a conversion affects pain meds, heart meds, or psychiatric drugs, double-check with your prescriber before making changes. Small math errors here can have big consequences. If you want, check related guides on this site about inhaler doses, antibiotics, and specific medicines to see real examples of how dosing equivalence plays out.

Tools that help: official drug monographs, manufacturer prescribing info, and trusted online calculators. For example, many albuterol MDIs deliver about 90 µg per puff — so two puffs equal ~180 µg on the label. Use those label values and ask a pharmacist before changing devices or dose counts. Keep dosing records handy.