Check on 23andme what type of CYP2D6 substrate metabolizer I am
Unless your polymorphisms have been tested against a particular drug, you can't extrapolate a starting dosage for it based only on your genotype.  It's better to actually take the drug, and change dosage based on observations.

See this paper

https://www.ncbi.nlm.nih.gov/pubmed/24935087


  CONCLUSIONS AND RELEVANCE: In this meta-analysis of randomized
  clinical trials, a genotype-guided dosing strategy did not result in a
  greater percentage of time that the INR was within the therapeutic
  range, fewer patients with an INR greater than 4, or a reduction in
  major bleeding or thromboembolic events compared with clinical dosing
  algorithms.


.


  why isn't this information more readily accessible to 23andme users,
  given the relatively large risks incurred by not knowing one's
  reaction to a large number of drugs?


Because no-way no-how does 23andMe want to be in the business of giving medical advice.  Nor should they be, because for most things, we have no idea how to convert genotype knowledge anything medically actionable.