Is there a reason not to practice jhana? From a chemical point of view, there has to be an after effect of the amount of joy and happiness one feels, right ? 

In a spiritual point of view, I know one should be aware of attachement to jhana. 

I am wondering because I experienced first jhana for the first time this week, and my natural tendency was to repeat this experience as much as I could in the following days. 
Is it advisable ?
Absolutely not! Go for it! (Assuming when you say first Jhana, you actually mean first Jhana and not something else). 


  "There is the case where a monk — quite withdrawn from sensuality, withdrawn from unskillful qualities — enters and remains in the first jhana: rapture and pleasure born from withdrawal, accompanied by directed thought and evaluation. He permeates and pervades, suffuses and fills this very body with the rapture and pleasure born from withdrawal. There is nothing of his entire body unpervaded by rapture and pleasure born from withdrawal.
  
  "Just as if a skilled bathman or bathman's apprentice would pour bath powder into a brass basin and knead it together, sprinkling it again and again with water, so that his ball of bath powder — saturated, moisture-laden, permeated within and without — would nevertheless not drip; just so, the monk permeates, suffuses and fills this very body with the rapture and pleasure born of withdrawal. There is nothing of his entire body unpervaded by rapture and pleasure born from withdrawal...