Concentration is the amount of drug/active ingredient in a given amount of product.
Drug concentration can be expressed in various ways, including percentage strength, parts per million, ratio strength and units. With regards to the percentage strength, this can be described in terms of percentage weight in volume, percentage weight in weight and percentage volume in volume.
This guide explores some of the ways that drug concentrations can be expressed in a medicine and explains the significance of each measurement.
Table of contents
- Percentage strength: Percentage weight in volume (% w/v)
- Percentage strength: Percentage weight in weight (% w/w)
- Percentage strength: Percentage volume in volume (% v/v)
- Parts per million (ppm)
- Ratio strength
- Units
Percentage weight in volume (% w/v)
The percentage weight in volume (% w/v) is the number of grams (g) of drug dissolved in 100 millilitres (mL) of liquid or solution.
A 2% w/v means there is 2 g of drug dissolved in 100 mL of liquid or solution.
A 4% w/v means there is 4 g of drug dissolved in 100 mL of liquid or solution.
Here are some examples of medicines that have their concentration expressed in % w/v, along with an explanation of what that concentration means and alternative ways to express this concentration.
| Medicine | What the concentration means | Alternative ways to express this % w/v |
| Alomide 0.1% w/v eye drops | There is 0.1 g of alomide in 100 mL eye drops | 0.1 g in 100 mL |
| 0.1 g/100 mL | ||
| Amorolfine 5% w/v medicated nail lacquer | There is 5 g of amorolfine in 100 mL nail lacquer | 5 g in 100 mL |
| 5 g/100 mL | ||
| Bupivacaine hydrochloride 0.5% w/v solution for injection | There is 0.5 g of bupivacaine in 100 mL injection | 0.5 g in 100mL |
| 0.5 g/100 mL | ||
| Physioneal 35 glucose 3.86% w/v / 38.6 mg/ml solution for peritoneal dialysis | There is 3.86 g of active ingredients in 100 mL solution | 3.86 g in 100 mL |
| 3.86 g/100 mL |
Percentage weight in weight (% w/w)
The percentage weight in weight (% w/w) is the number of g of drug in 100 g of the preparation.
A 6% w/w means there is 6 g of drug in 100 g of the preparation.
A 21% w/w means there is 21 g of drug in 100 g of the preparation.
Hydrocortisone cream and chloramphenicol eye ointment are two examples of medicines that have their concentration expressed as % w/w as shown below.
| Medicine | What the concentration means | Alternative ways to express this % w/w |
| Hydrocortisone 2.5% w/w cream | There is 2.5 g of hydrocortisone in 100 g cream | 2.5 g/100 g |
| 2.5 g in 100 g | ||
| Amorolfine 5% w/v medicated nail lacquer | There is 5 g of amorolfine in 100 mL nail lacquer | 5 g in 100 mL |
| 5 g/100 mL |
Percentage volume in volume (% v/v)
The percentage volume in volume (% v/v) is the number of mL of a drug in 100 mL of liquid or solution.
A 10% v/v means there is 10 mL of drug in 100 mL of liquid or solution.
A 15% v/v means there is 15 mL of drug in 100 mL of liquid or solution.
For example, desflurane’s concentration is expressed as % v/v as shown in the table below.
| Medicine | What the concentration means | Alternative ways to express this % v/v |
| Desflurane 100% v/v inhalation vapour liquid | There is 100 mL of desflurane in 100 mL vapour liquid | 100 mL/100mL |
| 100 mL in 100mL |
Parts per million (ppm)
1 part per million (ppm) means 1 in 1,000,000.
Some drugs have their concentrations expressed in parts per million. The parts per million can be weight in weight, volume in volume or weight in volume.
For example
Weight in weight: 1 g of drug in 1,000,000 g or 1 part of drug per million
Volume in volume: 1 mL of drug in 1,000,000 mL or 1 part of drug per million
Weight in volume: 1 g of drug in 1,000,000 mL or 1 part of drug per million
The concentration of fluoride in toothpaste can be expressed in parts per million. The table below shows the concentration of fluoride in ppm, what it means and the amount of fluoride found in 100 g and 1 g of toothpaste.
| Medicine | What the concentration means | In 100 g | In 1 g |
| Colgate Duraphat 2800ppm fluoride toothpaste | There are 2800 parts of fluoride per million or 2800 g of fluoride in 1,000,000 g | 0.28 g of fluoride | 0.0028 g or 2.8 milligrams (mg) of fluoride |
Ratio strength
Some drug concentrations are expressed as ratio strengths, such as 1 in 1,000,000.
Here are some examples of how a drug’s concentration can be represented in terms of ratio strength, along with an explanation of what that concentration means and the corresponding weight per mL.
| Ratio strength (weight in volume) | What the concentration means | Weight per mL |
| 1 in 1,000,000 | 1 g in 1,000,000 mL | 1 microgram (mcg) in 1 mL |
| 1 in 100,000 | 1 g in 100,000 mL | 10 mcg in 1 mL |
| 1 in 10,000 | 1 g in 10,000 mL | 100 mcg in 1 mL |
| 1 in 1000 | 1 g in 1000 mL | 1000 mcg in 1 mL |
| 1 in 100 | 1 g in 100 mL | 10 mg in 1 mL |
| 1 in 10 | 1 g in 10 mL | 100 mg in 1 mL |
| 1:100 | 1 g in 100 mL | 10 mg in 1 mL |
Adrenaline, also known as epinephrine, can be measured in terms of its concentration using ratio strengths. The table below illustrates the various ways in which the concentration of adrenaline can be represented.
| Adrenaline ratio strength | What this means in weight-volume terms | Weight of adrenaline per 10 mL | Weight of adrenaline per mL |
| 1 in 10,000 | 1 g of adrenaline in 10,000 mL | 1 mg | 0.1 mg |
| 1 in 1,000 | 1 g of adrenaline in 1,000 mL | 10 mg | 1 mg |
Units
Some drugs can be measured in units to indicate their concentrations.
The table below shows two examples of medicines whose concentrations are expressed in units: heparin injection and lantus injection in a vial.
| Medicine | What the concentration means |
| Heparin 5,000 I.U./mL solution for injection | There are 5000 units of heparin in 1 mL |
| Lantus 100 units/mL solution for injection in a vial | There are 100 units of insulin in 1 mL |
