We are all different. What wakes a happiness hormone in one might trigger a nasty and violent allergy in another. That is why we should always remember that what helps one person might hurt the other. Always consult your doctor for any medical advice or if you are in doubt.
Not all garlic side effects are known with the exception to obvious ones like garlic breath and garlic skin odour. Generally garlic is considered safe, however as with everything in life there are a few exceptions.
Digestive system irritation
The most common garlic side effects are stomach & digestive system-related and mostly caused by eating raw garlic, but this could also apply to garlic supplements:
Vomiting, diarrhoea, gas, burning in the mouth or/and throat, bad breath, body odour, skin irritation, blisters can also be side-effects.
Botulism is definitely the most dangerous of garlic side effects and it is not related to being allergic or intolerant to garlic in any form. It is strictly related to storing garlic.
Do not store raw garlic in oil, even for a few hours or after soaking it in vinegar. The botulinum toxin is the most potent toxin to human & animal kind.
Even if refrigerated, raw garlic stored in oil can cause potentially deadly botulism.
Some allium oil manufacturers will use acids (like vinegar) to eliminate the risks of Clostridium botulinum development, but they use additional processes which cannot be performed in your home kitchen to produce a garlic oil or garlic infused oil. http://en.wikipedia.org/wiki/Botulism
Other garlic side effects could include increased risk of:
bleeding or lowering the blood pressure, lowering the absorbance of some tuberculosis, aids/hiv drugs and birth control pills.
Intolerance or garlic allergy
Eating any quantities of garlic in any form might start an allergic reaction. It can be connected to sensitivity or intolerance to some or all garlic compounds. Some people might experience a mild irritation, others very severe reactions like swelling of the lips, tongue or throat, and problems with breathing etc. This could include any of the above-mentioned digestive system irritation symptoms, skin reactions etc.
For treatments and advice in regards to any of the above-mentioned garlic side effects always consult your doctor. In case of a severe reaction contact your local emergency services immediately.
A few references below from authority sources on garlic side effects: