Prime® is very safe and quite difficult to overdose to the point of harming tank inhabitants, but a large enough overdose can start to deplete the system of oxygen. This effect is temporary, typically lasting an hour at most, but in case of extreme overdose can be significant enough to harm fish. The standard dose of 5 mL per 50 gallons of water is recommended for standard tank setups, and an overdose of up to 5x this amount can be done to detoxify ammonia and nitrite in the water. However, there are a few situations under which a larger dose of Prime may not be safe for your system.
Your tank is already low in oxygen
A tank with an exceptionally high bio-load, high temperature, or poor circulation is likely quite low in oxygen, so reducing the oxygen any further may be dangerous to tank inhabitants. In this case, using a half dose of Prime® or increasing the oxygen content of the tank is recommended.
You are using another product that also reduces oxygen
Most conditioners that clear away chlorine and chloramine, products that detoxify ammonia and nitrite, as well as some medications will reduce oxygen levels in the tank. Like Prime®, the effect of these products on the oxygen level of the tank usually dissipates within an hour, meaning it is typically safe to use them on the same system as long as an hour or so is left between the doses.
For further information on which Seachem® products fall into this category, see FAQ: Can I use Prime® with ______?