It is possible, but very unlikely that you can develop an allergy to one or more of the ingredients in this product.
It is possible to have daytime sleepiness if the product is taken too late at night.
There is also the chance of increased dream activity as well as having vivid dreams because you will be spending more time in deep sleep.
Sleeping fewer hours, because you will be getting higher quality sleep you may find your self-waking up after only 4 or 5 hour, this however should not be considered a negative effect but you will have to gauge your response to your sleep patterns and how you feel.
Do not exceed the recommended dose, exceeding the recommended dose could cause a serotonin excess imbalance.