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To find out when a steady state can be reached, you need to know the half-life of a medication. Prozac has a half-life of 4-6 days. That means one pill taken today will be 50% washed out of the body in 4-6 days. So each pill taken between now and then adds
to the blood plasma level. I usually make a spreadsheet to help calculate it.
Let's assume the half-life is five days. That means each day you would lose about 7mg of that 60mg tablet. But each day you take a new tablet too. Day 1 you would have 60mg in your body. Day 2 you would have 113mg. Day 3 = 158. Day 4 = 195. Day 5 = 225. Day 6 = 248. Day 7= 263. Day 8 = 271. Day 9 = 272. Day 10 and on are also 272. So that would be a steady state. This gets more complicated with drugs that are metabolized into another chemical, which may have a different half-life than the original compound, and in which this second chemical may be the one doing the real work. In Prozac, for example, fluoxetine has the 4 - 6 day half-life, but it's metabolite norfluoxetine has a 9.3 day half-life. So it can take longer to reach plasma levels for that chemical. so that would mean 18 days to reach steady state. This is also further complicated if your body processes medications at different than average, such as kidney or liver damage which may allow the medication to hang around longer than the expected half-life, causing longer time to reach steady state and a higher steady state amount in the body.
Just because you reach steady state doesn't mean you have reached full potential or full reaction from a medication. Psychiatric medications are typically given 2-4 weeks before adjustments are made to allow for steady state to be reached, but also to allow the body to adjust to the chemical changes in the brain, and to allow the short-term negative side-effects to pass.
One other note, not all generics work the same, nor do generics always work as well as brand name. If you feel like the same dose is not working as well as a previous trial on the same medication, be sure to check that they were manufactured by the same company. The same medication may not give you the same reactions twice, because your body doesn't find it to be the same novel, unique, or intense change that it may have taken the medication to be previously. Kind of like the first bites of a cake taste sweeter than the following bites.
Hope that helps,