Psi-Drug - safe usage period?

[Garran]

The entry for Psi-Drugs in both the core and the CSC says...

A Traveller who takes more than one dose of Standard or Double Psi-Drug, or a single dose of the Special drug must make an END check, with DM-1 per dose of psi-drug taken in the last week.

Despite the ambiguous wording, I'm pretty sure that the intent here is that you can safely take a single dose of standard/double once a week? (Not once ever, and not once per indeterminate time period.)

[Moppy]

Seems logical (edit: And consistent with the other drugs).

Given the status of psi within the Imperium, I'd also think about how quality control on black market goods works.