What is a common side effect of doxycycline that patients should be warned about?

Doxycycline, a tetracycline antibiotic often used to treat a variety of infections, is well-known for its potential to cause photosensitivity as a common side effect. This condition makes the skin more sensitive to sunlight, leading to an increased risk of sunburn or skin rashes when exposed to UV rays. Therefore, it is crucial for patients to be aware of this risk, especially when they are spending extended time outdoors or in the sun.

While gastrointestinal distress can also occur with doxycycline, it is less commonly emphasized compared to the risk of photosensitivity, which can have immediate and visible effects on patients' skin. Dermatitis can also arise but is more associated with allergic reactions or sensitivity to the drug itself, rather than a side effect that most patients will universally experience. Weight gain is not typically associated with doxycycline use, making it less relevant when discussing common side effects.

Given this information, photosensitivity stands as the primary concern that patients need to be warned about before starting doxycycline treatment.