[CAL] Drug calculations

Question 1

How many mg per mL are in a 2% solution of lidocaine?

Answer 1

20 mg/mL

A 1% solution is equivalent to 1 g in 100 mL.
Therefore, 1000mg are in 100mL = 10mg in 1mL.

Question 2

What available tablet strengths are there for digoxin?

Answer 2

62.5 micrograms
125 micrograms
250 micrograms

Patient prescribed 187.5micrograms digoxin = 1 125microgram tablet + 1 62.5microgram tablet

Question 3

If a patient is on long term prednisolone, but is too unwell to take it orally, what can be give?

Answer 3

IM hydrocortisone

Remember to double to dose of prednisolone if acute illness

Question 4

What does 1% mean?

Answer 4

1% solutions have 1g of drug in 100mL

seen as either:

• 1g in 100mL
• or 10mg for 1mL (i.e. 1mL would contain 10 milligrams)

Question 5

How to calculate the volume needed to give a dose of drug?

e.g. 40mg of a drug produced in 80mg/2mL solution

Answer 5

Question 6

Hydralazine is presented as a 20 mg/2 mL ampoule.

how much NaCl is needed to be added to make a 1mg/1mL solution?

Answer 6

18ML

1mg/ml would be 20mg in 20mL so need to make up to that volume

Question 7

Are medications containing phenytoin base vs those with phenytoin sodium the same?

Answer 7

No

bioavailability is different with 100mg phenytoin sodium = 92mg phenytoin base

Question 8

What medications are affected by the so called “salt factor”?

Answer 8

• phenytoin
• digoxin
• sodium fusidate

Question 9

what is meant by the “salt factor” in relation to drug formulations?

Answer 9

Where formulations with a salt compound of the drug e.g. phenytoin sodium vs formulations with base version of drug have different doses to reach therapeutic effect

need to take care when converting from one to another

Question 10

orders to units

Answer 10

nanograms → micrograms → milligrams → gram → kilo

all by 1000