Abdur Rehman has been suspended for 12 weeks after testing positive for cannabis, the England and Wales Cricket Board announced on Wednesday.
"The club does not condone the use of illegal drugs in any circumstances and all Somerset players are made fully aware of this policy on a regular basis."
Rehman took 19 wickets in three Test matches against England last January when Pakistan swept the series 3-0 in Dubai and has a career haul of 81 wickets at an average loss of 28.
"I apologise to my family, the PCB, the ECB, Somerset County Cricket Club, my team-mates and my fans," said a remorseful Rehman.
"It was an error of judgement on my part that will cost me dearly and I would like it to be a lesson to all others in sports elsewhere.
"I will do my best to stay fit and focused during my suspension, and God willing, will be available for selection for the India series should the Pakistan Cricket Board see it fit for me to be selected."