You do not need to worry about overdosing when using transdermal magnesium. Upper limits for magnesium dosage are only set when using the product orally, as large doses can have a laxative effect due to it being a natural relaxant. This is why people take magnesium for muscle tension.
Toxic symptoms from increased magnesium intake are not common. This is because the body eliminates excess amounts, unless there are serious problems with kidney function. However, we always recommend speaking to your GP before starting any dietary supplements if you are on any medications or have any medical conditions. When you are using supplements, always adhere to nutrient reference value guidelines.