Is it a sin to lie to an addict and tell him that his medication is unavailable, out of fear of aiding his addiction or withholding the medication from a real patient?

Medicines are of two types: narcotics, which are not sold except with a medical prescription, and their use is permissible for dire need when no permissible alternative is available; and non-narcotics, for which the default is permissibility of sale, unless it is known or highly probable that the buyer is purchasing it for an unlawful purpose, in which case it should not be sold to him except with a medical prescription. Lying is forbidden except out of necessity or in cases where the Sharia has granted a concession. The way out is to use allusive speech (tawriya).