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.
While the Connecticut sales tax of 6.35% applies to most transactions, there are certain items that may be exempt from taxation. This page discusses various sales tax exemptions in Connecticut. In Connecticut, certain items may be exempt from the sales tax to all consumers, not just tax-exempt purchasers.