Doxycycline is used for malaria prophylaxis or treatment. When it is administered for treatment of P falciparum malaria, this drug must be used as part of combination therapy (eg, typically with quinine or quinidine).
If you buy goods and are not charged the Rhode Island Sales Tax by the retailer, such as with online and out-of-state purchases, you are supposed to pay the 7% sales tax (less any foreign sales tax paid) for these items yourself as the Rhode Island Use Tax.
Instead of taxing the sale of tangible property which takes place outside of Rhode Island's jurisdiction (and thus cannot be taxed), the Rhode Island Use Tax taxes the use or consumption of tangible property bought in other jurisdictions with a lower sales tax rate and brought back into Rhode Island.