AmGuard is very safe and quite difficult to overdose to the point of harming tank inhabitants, but a large enough dose can start to deplete the system of oxygen. The standard dose of 5 mL per 50 gallons of water is recommended for general ammonia control, and higher doses can be calculated to deal with elevated ammonia levels (always dose according to a known level of ammonia in the system). There are a few situations where caution should be taken with elevated doses though:
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. For further information on which Seachem products fall into this category, see FAQ: Can I use Amguard with ______?