Some old studies suggested that the ingestion of large doses of thiamine (vitamin B) could be effective as an oral insect repellent against mosquito bites. However, there is now conclusive evidence that thiamin has no efficacy against mosquito bites. Some claim that plants such as wormwood or sagewort, lemon balm, lemon grass, lemon thyme, and the mosquito plant (Pelargonium) will act against mosquitoes. However, scientists have determined that these plants are "effective" for a limited time only when the leaves are crushed and applied directly to the skin.
There are several, widespread, unproven theories about mosquito control, such as the assertion that vitamin B, in particular B (thiamine), garlic, ultrasonic devices or incense can be used to repel or control mosquitoes. Moreover, manufacturers of "mosquito repelling" ultrasonic devices have been found to be fraudulent, and their devices were deemed "useless" according to a review of scientific studies.