The SR5 addiction text wording is, to say the least, nearly the worst written ruleset I have ever tried to analyze. (Some language translations are worse, because the interpreter was not a gamer.) Ultimately, I could not see how to resolve it all with literal interpretation.
The worst part for me is where a later sentence states, "This means that substances with high addiction ratings (like kamikaze) could get you hooked in a single dose." There is no interpretation where that can happen...
"Every time you use an addictive substance during X weeks in a row you need to make an Addiction Test."
The technical interpretation of that is that you have to wait X weeks, and if in any week of the X period, you did not use the substance, you do not have to make even a single Addiction test. The If qualifier is using the substance X weeks in a row.
For soycaf, X is 9. So you can take Soycaf 8 weeks out of 9, and never make a single test. But if you use it in 9 straight weeks, you now make how many test? One for every time you used... did you remember to track that? That is a minimum of NINE tests, all of which wait until the ninth week, because the qualifier is not true until Week 9.
But this leads to another absurdity. You can take Soycaf 1000 times each in weeks 1 through 8 (8000 uses total), and so long as a full week passes with no use, you make ZERO tests. You did not use nine weeks in a row, so do not need to make a single test.
So, for kamikaze, X is 2, meaning you can use it every other week and never make a single Addiction test. That does not conform to the stated intent for high addiction substances. X needed to be 1 for that. But as is, with 2, you can use kamikaze 20 times in Week 1, never in Week 2, and not make a single Addiction Test.
"The clock on this keeps ticking even if you skip a week"
There is no defined clock. Nine weeks in a row is nine weeks in a row, not nine weeks out of ten, with a random non-use week in the middle. Going 8 weeks in a row, skipping a week, then using again does not create 9 weeks in a row. It is 8 weeks in a row and then a new 1 week in a row. The qualifier is not "Nine weeks out of ten" or eleven or twenty. This sentence is nonsensical, because there is no clock that can be defined as "ticking". There is no interpretation of this sentence that does not lead to a new absurdity.
"but every week you go without indulging reduces the Addiction Threshold by 1".
Okay, but if I don't use during a week, I make no test that week, so why does this matter? Tests result from using X weeks in a row, and that ended when I skipped a week. No test until I use X weeks in a row again. But the best is:
"it returns to normal when you use the substance again"
Okay, so after skipping 4 weeks, I restart using, use for X weeks in a row, and now make X Addiction tests. What is the Threshold? Full threshold. The threshold was reset to full in Week 1 and has stayed there. All the rolls are in Week X. There is, at no point, an Addiction Test that could be affected by the described reduced threshold.
There is a weird interpretation where at the end of week X, you make the X Addiction Tests, but the one in week 1 now happened retroactively. The problem with that is the character may have been addicted in Week 1, but if the run in Week 2 did not include Addiction effects, the run was played wrong. The tests cannot be retroactive. You cannot fix this by rolling the test each week and waiting to see if use lasts for X weeks, because when the player sees the positive Addiction result, he simply skips using Week X-1 to ensure it cannot be applied.
There really is no resolution. The intent cannot be achieved by the letter of the rules. You have to house rule it. Theproblem is "When do you make a test?" You can redefine how often based on your own idea of what makes sense... but beware. Addiction in SR5 destroys characters. Saving the character from burnout requires spending karma.. and that karma does not make the character more powerful: it staves off decay.Other, non-addict characters, are growing. Your addict is telling a story that happens on our streets every day. If you are playing RPGs to escape reality, and most do, addiction is all too real in SR5.
Fixed in SR6. Addiction cannot get worse, except willingly with DM agreement and should result in positive karma expenditure.