Over 100 substances, swallowed or applied to the skin, are known to cause sun-induced reactions on the skin. Here is a list of the top 30 substances that sensitize the skin to sunlight. All of them can cause photosensitivity of two types: phototoxicity (similar to a sunburn and occurs after swallowing certain drugs, ony on sun exposed area) and photoallergy (an allergic rash with blisters and hives, can also affect areas not exposed to the sun).
Antianxiety drugs
Alprazolam
Chlordiazepoxide
Antibiotics
Quinolones
Sulfonamides
Tetracyclines
Trimethoprim
Antidepressants
Tricyclic antidepressants
Antifungal drugs (taken by mouth)
Griseofulvin
Antihyperglycemics
Sulfonylureas
Antimalarial drugs
Chloroquine
Quinine
Antipsychotics
Phenothiazines
Chemotherapy drugs
Dacarbazine
Fluorouracil
Methotrexate
Vinblastine
Diuretics
Furosemide
Thiazides
Drugs used to treat acne (taken by mouth)
Isotretinoin
Heart drugs
Amiodarone
Quinidine
Pain-relief drugs (analgesics)
NSAIDs (especially piroxicam and ketoprofen)
Skin preparations
Antibacterials (such as chlorhexidine and hexachlorophene)
Coal tar
Fragrances
Furocoumarin-containing plants, such as limes, celery, and parsley
Sunscreens
If you don’t take anything on this list, the most common condition causing a sun rash is called Polymorphous Light Eruption or PMLE. It is known to affect more than 10% of north americans. You can find pictures of PMLE rash here and my top 6 tips to prevent PMLE.