When you take Bactrim, a combination antibiotic made of sulfamethoxazole and trimethoprim, often prescribed for urinary tract infections, bronchitis, and traveler’s diarrhea. It’s known for its effectiveness—but also for a hidden risk: sulfa drug photosensitivity. This isn’t just a mild rash. Bactrim can turn a normal day outside into a painful, blistering sunburn—even if you’ve never burned before.
Why does this happen? Bactrim belongs to the sulfonamide class of antibiotics. These drugs absorb UV light, triggering a chemical reaction in your skin that damages cells. The result? Redness, swelling, itching, and sometimes peeling or blisters. It doesn’t matter if you’re in the shade or wearing sunscreen—this reaction can still happen. People on Bactrim for more than a few days are especially at risk, and the effect can last for days after you stop taking it. This isn’t rare. Studies show up to 1 in 10 people on sulfonamides report some level of sun sensitivity. And if you’re fair-skinned, older, or already taking other medications like diuretics or NSAIDs, your risk goes up even more.
It’s not just about avoiding the beach. Even walking to your car, sitting by a window, or working outside during lunch can be enough to trigger a reaction. You don’t need to be on vacation to get burned. That’s why the advice isn’t just "use sunscreen"—it’s "stay out of direct sunlight as much as possible." Wear long sleeves, a wide-brimmed hat, and UV-blocking sunglasses. Use broad-spectrum SPF 50+ every single day, even if it’s cloudy. And never assume you’re safe just because you didn’t burn last time. This reaction can get worse with each exposure.
What if you’ve already burned? Stop taking Bactrim only if your doctor tells you to. Don’t quit your antibiotic cold turkey—it could make your infection worse. Instead, call your provider right away. They might switch you to a different antibiotic or suggest ways to manage the reaction. In the meantime, treat the burn like any severe sunburn: cool compresses, aloe vera, and plenty of water. Avoid popping blisters. If you feel dizzy, have a fever, or the skin starts oozing, get to a clinic fast. This isn’t just a cosmetic issue—it can lead to infection or long-term skin damage.
Other antibiotics like tetracycline, doxycycline, and ciprofloxacin carry similar risks. But Bactrim is one of the most common culprits because it’s prescribed so often. If you’ve ever gotten a bad sunburn after starting a new antibiotic, write it down. Share it with your doctor next time you’re prescribed anything. You’re not overreacting—you’re protecting your skin. The posts below cover real cases, what doctors actually recommend, and how to spot early signs before it turns dangerous. You’ll also find what to do if you’re on Bactrim and need to be outside, and which other drugs make this risk worse.
Doxycycline and TMP-SMX can cause severe sun sensitivity, leading to painful burns even with brief exposure. Learn how to protect your skin, which sunscreens work, and how long the risk lasts after stopping these antibiotics.