Most reports of insect repellent toxicity relate to DEET, or N,N-diethyl-3-methylbenzamide, the major synthetic agent in common use. DEET is considered safe when used as recommended; most serious problems have resulted from inappropriate use or extreme exposure. In the 52 years that DEET has been in widespread use, fewer than 50 confirmed cases of serious DEET toxicity have been reported. In many of these cases, details of exposure were not well documented, making it difficult to establish cause. That said, there remains concern about long-term safety of DEET. Scorecard.org lists it as a suspected toxin affecting multiple organ systems.
Topical reactions are the most common, generally manifested as redness and itching. There may be small vesicles (blisters containing clear fluid), similar to a poison ivy/poison oak reaction. These can be treated with over-the-counter corticosteroid cream and are self-limited. There has been one case of a more serious anaphylactic reaction from skin contact. Eye irritation from accidental spraying of DEET into the eyes is common but not serious, and generally needs no treatment beyond irrigation of the eyes.
Systemic, cardiovascular, and GI effects
Systemic reactions such as fatigue, weakness, and muscle aches and pains are rare but have been reported with DEET. These have resolved without treatment. Cardiovascular reactions such as hypotension (low blood pressure) and bradycardia (abnormal slowing of heart rate) are extremely rare---as of 2009 only one case had ever been reported. Severe hypotension or bradycardia can lead to decreased blood flow to the brain and other organs. GI symptoms of DEET toxicity include nausea, vomiting and diarrhea.
Neurological toxicity is the most serious danger, and can include memory loss, headaches, seizures, tremors, gait disturbance and encephalopathy. Coma, seizures and death have resulted in three cases where large amounts of high-concentration DEET preparations were deliberately ingested.
Cancer and birth defects
Laboratory studies have shown no increased risk of cancer or birth defects. DEET use is considered safe for pregnant and breast-feeding women.
DEET can reduce the efficacy of sunscreen when the two are used together. However, the FDA recommends that sunscreen and insect repellent continue to be used together; if necessary, reapply the sunscreen more frequently. Products containing both sunscreen and insect repellent should be avoided as repellent doesn't require reapplication as frequently as sunscreen.
Keep insect repellents away from eyes, mucous membranes, open wounds and irritated skin. Avoid inhaling vapors from sprays. The American Academy of Pediatrics advises against the use of DEET on children under 3 years old, and sticking with low concentrations (maximum 10%) for older children. Adults should not use concentrations over 30% except in extreme situations. Up to 10% of topically applied DEET may be absorbed; this is cleared from the body by the kidneys after being metabolized so those with impaired organ function should consult a doctor before using DEET. Potential for adverse reaction may be increased when DEET and other insect repellents, such as permethrin, are used together.