The most serious side effect of too much insulin is hypoglycemia, or low blood glucose. Hypoglycemia can be a life-threatening, even fatal condition.

Classic signs of hypoglycemia

• lethargy (lack of energy)
• weakness
• head tilting
• "drunkedness" - wobbling when walking, unbalanced
• hunger
• restlessness
• shivering
• ataxia - usually lack of muscular coordination, but maybe changes in head and neck movements
• disorientation
• stupor
• convulsions or seizures
• coma

The occurrence of signs depends on how far the bg drops and on how fast the blood glucose drops.

Owners of diabetic cats have also reported observing these signs

• sleepiness
• unable to wake the cat easily when it is sleeping.
• vomiting
• glassy eyes - it may look like it is staring into space
• laying, sleeping, or curled up in an unusual location of the house
• meowing, crying, yowling, or vocalizing in a way that is unusual for your cat
• some cats get aggressive
• drooling
• coughing

Owners of diabetic dogs have also reported observing these signs

• sweating - check the nose and the paw pads.
• lip smacking or licking
• getting physically "stuck" in a place where the pet normally could get itself out (for example, behind a partially closed door that a pet would usually nudge open.)