Questions: Order: Famotidine (Pepcid) 20 mg IV q12h. Available: Famotidine 40 mg / 4 ml vial.

To administer the prescribed dose of Famotidine (Pepcid) 20 mg IV every 12 hours, we need to determine the volume of the solution to be used from the available vial concentration.

Available concentration: Famotidine 40 mg/4 ml

Prescribed dose: 20 mg

To find out how many milliliters are needed for a 20 mg dose, we can set up a proportion based on the available concentration:

\[ \frac{40 \text{ mg}}{4 \text{ ml}} = \frac{20 \text{ mg}}{x \text{ ml}} \]

Solving for \(x\):

1. Cross-multiply to solve for \(x\): \[ 40 \text{ mg} \times x \text{ ml} = 20 \text{ mg} \times 4 \text{ ml} \]

2. Simplify the equation: \[ 40x = 80 \]

3. Divide both sides by 40 to solve for \(x\): \[ x = \frac{80}{40} = 2 \text{ ml} \]

Therefore, to administer a 20 mg dose of Famotidine, you need to use 2 ml of the solution from the vial.

Summary: You should draw up 2 ml from the Famotidine 40 mg/4 ml vial to administer the prescribed 20 mg dose IV every 12 hours.