This invention in one embodiment is directed to a compound of Formula Ia; where the designation (R) indicates that the designated carbon has the (R) stereochemistry; and wherein Z1 is hydrogen or fluorine; Z2 is hydrogen, deuterium, or fluorine; Z3 is deuterium; Z4 is fluorine; m is an integer from 0 to 10; n is an integer from 0 to 2; provided that: the sum of m+n does not exceed 10; and when both Z1 and Z2 are hydrogen, the sum of m+n is greater than 0, and pharmaceutically acceptable salts thereof. This invention also provides compositions comprising one or more compounds of this invention and a carrier and the use of the disclosed compounds and compositions in methods of treating diseases and conditions that are beneficially treated by administering an antihelminthic agent, such as praziquantel.