TOPIC:

So a patient asked me this question - "Should I wait 6 months after treatment before trying to get pregnant or would 3 months do?"

Being a little on the Aspberger spectrum I thought that's interesting, let's calculate the answer:

Hello T,

Drugs exit body at a rate determined by what is known at their 1/2 life.

Sofosbuvir has half life of 0.4 hours but main metabolite GS-331007 has half life of 27 hours (call it 1 day)

Ledipasvir has a slower half life of 47 hours (call it 2 days) meaning that level will fall to 1/2 after 2 days.

Let's forget about Sofosbuvir for a moment and consider how long the Ledipasvir will take to fall

So over 1 month ledipasvir levels will fall

Original x 1/2 x 1/2 x 1/2 x 1/2 x 1/2 x 1/2 x 1/2 x 1/2 x 1/2 x 1/2 x 1/2 x 1/2 x 1/2 x 1/2 x 1/2 = 0.00003 = 3e-5 = 0.003%

After 2 months 0.0000000009 (9e-10)

After 3 months 0.00000000000003 (3e-14)

After 4 months 9e-19

After 5 months 3e-23

After 6 months 8e-28

So very little Ledipasvir will be left after 3 months. For sofosbuvir GS-331007 metabolite the level will be about 8e-28 at the same time (because it exits twice as fast, so it's the same number that Ledipasvir reaches in twice the time).

Now The number 6 x 10^23 (6e23) is called Avogadro's number. It's an interesting number because if I have:

• 529g of Sofosbuvir I have what is known in chemistry as 1 mole and it contains Avogadro's number of single Sofosbuvir molecules
• 889g of Ledipasvir is also 1 mole

Now the significance of Avogadro's number and the half life decay is that when I exceed 6 x 10e-23 we are literally down to the last molecule (assuming we started with 1 mole).

Now we don't start with one mole of either Sofosbuvir (34/529 moles) or Ledipasvir (8/889 moles) because we have about 34g total Sof dose and 8 g total Led dose.

For Sofosbuvir there will be nothing left within 3 months. For Ledipasvir there will still be a little, but there will definitely be nothing at 6 months.

So for complete safety 6 months would be best.

In fact because we only start with 8/889 ~ 1/100 = 1e-2 at 5 months we will have 3e-23 x 1e-2 = 3e-25

2 days later we will have 1.5e-25
4 days later we will have 7.5e-26
6 days later we will have 3.75e-26 and less than Avogadro's number of molecules

So more precisely you should wait 5 months and 6 days

Given you ovulate 10-14 days after the start of your menstrual period you would be safe to wait 5 months and then start trying with the optimal time being that 10-14 day period.

Now I know why I'm not a doctor....

But if RBV is in the mix it would be significantly longer than six months! James, over to you on that one.

The half life of Riba is 12 days so we would need 6 times as long as the Ledipasvir - about 36 months

The rule of thumb would be 3 months per day of half life.

So don't want to go off subject too much,but as far as some of the supplements or foods I stopped taking during sof/dac tx, can I start taking these again at EOT? Or is it better to wait a while?

According to this article, we each have about 32.7 Trillion cells in our bodies.

www.smithsonianmag.com/smart-news/there-...body-4941473/?no-ist

Now, 32.7 trillion = 32.7 * 10^12. So if you are thinking about how much of an unwanted drug molecule you might want to have in your body, wouldn't it be reasonable to say you're pretty safe if you get down to less than about one molecule per cell?

Or in terms of moles, your pretty clean once you get down to about 6.023*10^23 / 3.27*10^13 = 2*10^-10 = 20 nano-moles?

Me, I'm pretty sure I've got nanomoles of all kinds of crap in my body, but it probably isn't doing any harm (and I don't believe in homeopathy).

So wouldn't this be a "safe enough" point at which to break out the champagne and have an early night?

[... Just a thought, since getting pregnant is physically impossible for me as I am only a cartoon character... ]