Herb Tulsi (Holy Basil) has many benefits and advantages but has several side effects also, it is not recommended for all. Below is a list of top precautions of Tulsi and who should not take it:
1. It has anti fertility effect: Pregnant women and breast feeding mothers should not take it. Couples trying for children should not take it.
2. It is a blood thinner: Tulsi has blood thinning effects, if blood thinning can cause problem to you, you should not take Tulsi e.g. if you are going for a surgical operation
3. It lowers blood glucose: Diabetes patients should take it carefully as it reduces blood glucose.
It should always be taken in limited quantities after consultation with a practicing ayurvedic doctor.
References: http://www.livestrong.com/article/136596-side-effects-holy-basil/