Nytol, also known as Benadryl in the United States contains diphenhydramine, which is an anti-histamine that is commonly used over-the-counter to treat allergies, insomnia, motion sickness, and itching. Diphenhydramine works by competing with histamine at the histamine receptors, leading to a decrease in symptoms caused by histamine release such as itching, allergic reactions, runny nose, watery eyes, and increased acid secretion in the stomach. Diphenhydramine can also cause drowsiness and for this reason is often used as a sleep aid.
Common side effects associated with diphenhydramine include dry mouth, dizziness, confusion, and difficulty urinating. According to the Beer's Citeria, diphenhydramine use should be avoided in patients who are greater than 65 years old because of an increased risk of the development of these side effects.
Ritalin (methylphenidate) is a stimulant that is used to treat attention deficit hyperactivity disorder (ADHD), narcolepsy, and fatigue. It works by increasing the amount of norepinephrine and dopamine that is available outside of your neurons in a similar manner as amphetamines. Because of it's similarity to amphetamines, methylphenidate has a high risk for abuse and dependence. Side effects of methylphenidate include headache, insomnia, irritability, nausea, decreased appetite, increased heart rate, and dry mouth.
Diphenhydramine and methylphenidate are safe to take together and there are no drug interactions associated with concomitant use. They do however, have conflicting actions (stimulant effect vs. sedative effect) and may potential offset each other to a degree. As always, it is important to talk to your doctor or pharmacist before taking new medication to ensure they are safe to take together.