Now is provided a new sustained release drug preparation comprising such an inclusion complex of a medical compound with a hydrophobic cyclodextrin derivative, e.g. ethylated cyclodextrins, which sustains or retards the dissolution and release of the medical compound at a controlled rate from the inclusion complex and hence from the drug preparation containing the inclusion complex, so as to maintain the concentration of the medical compound in blood at an effective level for prolonged time. This drug preparation may further contain a second type of an inclusion complex of the medical compound with a hydrophilic cyclodextrin or hydrophilic cyclodextrin derivative, in mixture with the first type of the inclusion complex of the medical compound with a hydrophobic cyclodextrin derivative, to maintain the controlled release of the medical compound from the drug.