Before his sacking from Advisory Council On The Misuse of Drugs, David Nutt and the committee suggested that a classification for new/little researched drugs were should be created. Class D drugs would be regulated (e.g. sold only to over 18s) and their supply and usage monitored to assess their potential harm.
The alternatives are to take a cautious approach and ban them outright, or leave them unregulated.
The drug classification system in the UK is divided into A (e.g. heroine Ecstasy), B (Amphetamines, Cannabis) and C (GHB, Ketamine). Higher grades correspond to increased punishment for possession and supplying and (supposedly) a higher risk of harm for the substance.