Orphenadrine (Norflex®) is also useful at relieving the pain associated with muscle sprains, strains, and injuries. It is available in tablet form (conventional or extended-release) and is typically taken twice per day. As with the other muscle relaxants described above, orphenadrine has the potential for negative drug interactions therefore patients should provide a list of current and planned medications (prescription and non-prescription) when discussing therapy with their doctor. It is especially important to mention the following medications: amantadine (Symadine®), fluphenazine (Prolixin®), haloperidol (Haldol®), depression medications, perphenazine (Trilafon®), prochlorperazine (Compazine®), promethazine (Phenergan®), and trifluoperazine (Stelazine®). In addition, individuals who have a history of glaucoma, myasthenia gravis, ulcers, urinary tract or intestinal blockages, enlarged prostate, irregular heartbeat, or liver, kidney or heart disease should discuss their options carefully, as orphenadrine may or may not be the most appropriate therapy. Common side effects associated with the use of orphenadrine include dry mouth, drowsiness, dizziness, lightheadedness, upset stomach, vomiting, constipation, difficulty urinating, blurred vision, and headache. Serious side effects include fast or irregular heartbeat, fainting, confusion, hallucinations, and skin rash.
The influence of renal impairment on the pharmacokinetics of haloperidol has not been evaluated. About one-third of a haloperidol dose is excreted in urine, mostly as metabolites. Less than 3% of administered haloperidol is eliminated unchanged in the urine. Haloperidol metabolites are not considered to make a significant contribution to its activity, although for the reduced metabolite of haloperidol, back-conversion to haloperidol cannot be fully ruled out. Even though impairment of renal function is not expected to affect haloperidol elimination to a clinically relevant extent, caution is advised in patients with renal impairment, and especially those with severe impairment, due to the long half-life of haloperidol and its reduced metabolite, and the possibility of accumulation (see section ).