A method of preparing a pharmaceutical composition having a drug-containing core enclosed by one or more metal oxide materials is provided. The method includes the sequential steps of (a) loading the particles comprising the drug into a reactor, (b) applying a vaporous or gaseous metal precursor to the particles in the reactor, (c) performing one or more pump-purge cycles of the reactor using inert gas, (d) applying a vaporous or gaseous oxidant to the particles in the reactor, and (e) performing one or more pump-purge cycles of the reactor using inert gas. The temperature of the particles does not exceed 35° C. This produces a pharmaceutical composition comprising a drug containing core enclosed by one or more metal oxide materials.