A female client with a urinary tract infection is prescribed co-trimoxazole (trimethoprim-sulfamethoxazole). Which instruction should the nurse provide?

• A. Take the medication with food.
• B. Drink at least eight 8-ounce glasses of fluid daily.
• C. Avoid taking antacids during co-trimoxazole therapy.
• D. Don't be afraid to go out in the sun.

Answer: (B)

Hydration helps prevent crystalluria when taking trimethoprim-sulfamethoxazole for a urinary tract infection. This drug can crystallize in urine if fluid intake is too low, which can irritate the kidneys or lead to stones. Drinking plenty of fluids increases urine flow and helps keep the drug dissolved and eliminated more safely.

That’s why the nurse should emphasize drinking at least eight 8-ounce glasses of fluid daily, ideally with a full glass of water with each dose. While taking with food can ease stomach upset and photosensitivity is a consideration (so sun protection is wise), the most protective and actionable instruction for this medication is maintaining good hydration to prevent crystal formation and renal irritation.