Imipramine and trihexyphenidyl both cause "drying" side effects. Taking them together adds these effects up, so they can be stronger than with either medicine alone. You may notice a dry mouth, blurred vision, constipation, or difficulty passing urine.
These medicines are often taken together with some care. Your doctor may change a dose, space them apart or check on you more often. Make sure your doctor and pharmacist know you take both.