Any emergence of mood changes (irritability or dysphoria) or agitation is very relevant and should be reported to the doctor!
There a many possible causes that could contribute to this change.
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Titration of the right dose
The medication needs to be monitored and adapted to the personal needs. So the child might need a higher dose of the medication or might be very sensitive to psychostimulants. This should be evaluated!
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Rebound
Normal methylphenidate pills work for about 3 to 4 hours. After that time a "rebound" of the old symptoms can occur. The patients feel very uncomfortable with this. Adaptation of the right dose and interval is necessary or you might try long-acting psychostimulants instead.
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Change stimulant preparation
Some children do not respond to methylphenidate but do fine with amphetamines.
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Evaluate comorbid disorders like depression or anxiety disorders!!!
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Assess sleep.
Many adhd children have severe problems at night. If they cannot sleep they might feel irritable and dysphoric.