Any addict, who is charged with an offence punishable under section 27 or with offences involving small quantity of narcotic drugs or psychotropic substances, who voluntarily seeks to undergo medical treatment for de-addiction from a hospital or an institution maintained or recognised by the Government or a local authority and undergoes such treatment shall not be liable to prosecution under section 27 or under any other section for offences involving small quantity of narcotic drugs or psychotropic substances:
Provided that the said immunity from prosecution may be withdrawn if the addict does not undergo the complete treatment for de-addiction.]