When one stops Zoloft too rapidly a withdrawal syndrome may develop. Among the symptoms that may be experienced are nausea, tremors, lightheadedness, muscle pains, weakness, insomnia, and anxiety. The withdrawal symptoms usually last 1-2 weeks but in some instances they may gradually decrease over a period as long as a month. It is generally recommended that one taper off this medication gradually under a doctor's supervision. Check with your own personal physician for specific instructions.

From: http://depression.about.com/cs/sidee...aoiszoloft.htm

To abruptly stop Valium after an extended period of use is extremely dangerous and can cause seizures and sometimes death. Discontinuation of the medication must include a physician supervised gradual taper schedule and/or adjunct medications to minimize acute withdrawal.

Essentially, withdrawal symptoms from Valium are like the mirror of its therapeutic effects. Valium withdrawal can produce especially severe withdrawal symptoms similar to those in alcohol and barbiturate withdrawal, including jittery, shaky feelings and any of the following: rapid heartbeat, tremor, insomnia, sweating, irritability, anxiety, blurred vision, decreased concentration, decreased mental clarity, diarrhea, heightened awareness of noise or bright lights, impaired sense of smell, loss of appetite, loss of weight, muscle cramps, seizures, tingling sensation, and agitation. In more extreme cases, typically associated with sudden cessation of the drug, users may experience convulsions, tremor, abdominal and muscle cramps, vomiting and sweating.

From: http://www.drugabusehelp.com/drugs/valium/