According to http://products.sanofi-aventis.us/plavix/plavix.html " Plavix® has been evaluated for safety in more than 17,500 patients, including over 9,000 patients treated for 1 year or more."

There are several side effects that have been shown for clopidogrel :

The main effect observed is that while clopidogrel is being taken it may take longer for bleeding to stop if the patient injures themselves. The patient may also bruise more easily as a result of of taking clopidogrel. The number of platelets (thrombocytopenia) and the number of white blood cells (neutrophils) decrease whilst taking clopidogrel.
 

Several serious but unlikely side effects include

- Confusion

- Slurred Speech

- Vision Changes

- Chest pain

- Swelling

- Depression

Other noted side effects include

- Stomach upset

- Diarrhoea

- Constipation

- Headache

- Rash

It is possible though highly unlikely that the patient may suffer a serious allergic reaction through taking clopidogrel. This might include a severe rash and trouble breathing.

An overdose of clopidogrel may result in extended bleeding time. http://products.sanofi-aventis.us/plavix/plavix.html states that a single oral dose of clopidogrel at 1500 or 2000 mg/kg was lethal to mice and to rats and at 3000 mg/kg to baboons.