AKG supplementation alone doesn’t appear to carry significant side effects. One study found the no observed adverse effect level (NOAEL) to be at 1g/kg body weight in rats.[1]
Adverse effects, including heart palpitations and dizziness, have been reported by those using AAKG, which is a combination of arginine with AKG. This combination is commonly used by athletes.[2][3]
While AAKG is considered to be ergogenic, the evidence to support this is actually not conclusive. Improvements in strength have been reported in some studies,[4] but other studies report no improvement in performance, strength, and endurance.[3][5] Given the potential for adverse effects, the risk might not be worth the small potential benefits.