Question

I need to keep 20.0 mg of a drug in my system at a minimum and a Maximum of 140. mg. The half life of the drug that I put in my system is 16 hours. I have put 100.0 mg in my system by ingesting a pill. When is the earliest I should take the next 100.0 mg pill? When is the latest?

Answer 1

Therefore, the earliest time to take the next pill is after 10.85 hours and the latest time is before 15.14 hours have passed.

Explanation:

The concentration of the drug in your system after ingesting 100.0 mg can be calculated using the formula:

C = D / V

where C is the concentration, D is the dose, and V is the volume of distribution (assumed to be constant).

At the initial time, t = 0, the concentration is:

C(0) = 100.0 mg / V

After one half-life, t = 16 hours, the concentration is:

C(16) = C(0) / 2 = 50.0 mg / V

After two half-lives, t = 32 hours, the concentration is:

C(32) = C(16) / 2 = 25.0 mg / V

After three half-lives, t = 48 hours, the concentration is:

C(48) = C(32) / 2 = 12.5 mg / V

To find the earliest and latest times to take the next 100.0 mg pill, we need to calculate the concentration at those times and make sure it stays between 20.0 mg and 140.0 mg.

Let's first find the earliest time at which the concentration drops below 20.0 mg.

20.0 mg / V = C(16 + x) = 50.0 mg / V / (2^(x/16))

Solving for x:

2^(x/16) = 50.0 / 20.0 = 2.5

x/16 = log2(2.5)

x = 16*log2(2.5) = 16*0.678 = 10.85

So the earliest time to take the next pill is after 10.85 hours.

Now let's find the latest time at which the concentration goes above 140.0 mg.

140.0 mg / V = C(16 + x) = 50.0 mg / V / (2^(x/16))

Solving for x:

2^(x/16) = 50.0 / 140.0 = 0.3571

x/16 = log2(0.3571)

x = 16*log2(0.3571) = -15.14

So the latest time to take the next pill is before 15.14 hours have passed.