The probability of a false positive result = probability of not doing meth * probability of getting a positive test result while not doing meth. P(+ ∩ M') = P(M') * P(+|M').
That pretty clearly depends on the prior probability, P(M).
That pretty clearly depends on the prior probability, P(M).
It doesn't matter what the prior probability is of a random person having used meth (or whatever you are testing for); it only matters how likely it is that someone who has not used meth will be given a positive result.