The Science Select Committee said the present system was based on historical assumptions, not scientific assessment.
BBC News has seen details of a system devised by government advisers which was considered by former Home Secretary Charles Clarke but is now on hold.
It rates some illegal drugs as less harmful than alcohol and tobacco.
MOST HARMFUL DRUGS
Drugs Top 20
- Heroin
- Cocaine
- Barbiturates
- Street Methadone
- Alcohol
- Ketamine
- Benzodiazepines
- Amphetamines
- Tobacco
- Buprenorphine
- Cannabis
- Solvents
- 4–MTA
- LSD
- Methylphenidate
- Anabolic Steroids
- GNB
- Ecstasy
- Alkyl nitrites
- Khat
CURRENT DRUG CLASSIFICATION
Class A
Cocaine/crack
Heroin
Ecstasy
LSD
Magic mushrooms
Crystal meth (pending)
Class A/B
Amphetamines
Class C
Cannabis
Ketamine
Source: BBC News