Source: Medicines & Healthcare Products Regulatory Agency (GB) Revision Year: 2018 Publisher: Genus Pharmaceuticals Holdings Limited, T/A Genus Pharmaceuticals, Linthwaite, Huddersfield, HD7 5QH, UK
Trihexyphenidyl is an antispasmodic drug which exerts a direct inhibitory effect on the parasympathetic nervous system. It also has a relaxing effect on smooth muscle.
It is indicated in all forms of Parkinsonism (postencephalitic, arteriosclerotic and idiopathic). It is often useful as adjuvant therapy when treating these forms of Parkinsonism with levodopa. Trihexyphenidyl is effective in reducing the rigidity of muscle spasm, tremor and excessive salivation associated with Parkinsonism. Trihexyphenidyl is also indicated to control extrapyramidal disorders (eg akathisia manifested by extreme restlessness and dyskinesia characterised by spastic contractions and involuntary movements) due to central nervous system drugs such as reserpine and the phenothiazines.
Optimal dosage should always be determined empirically, usually by initiating therapy at a relatively low level and by subsequent graduated increments.
The usual dosage for Parkinsonism is 6-10mg per day although some patients chiefly in the post-encephalitic group may require an average total dose of 12-15mg daily. It should be given orally either three or four times a day at mealtimes.
Normal dosage for drug-induced Parkinsonism is usually between 5mg and 15mg per day, although some cases have been controlled by 1mg daily.
In all cases, trihexyphenidyl dosage should be increased or decreased only by small increments over a period of several days. In initial therapy the dose should be 1mg the first day, 2mg the second day with further increases of 2mg per day at three to five-day intervals until the optimum dose is reached.
If patients are already being treated with other parasympathetic inhibitors, trihexyphenidyl should be substituted as part of the therapy. When trihexyphenidyl is used concomitantly with levodopa the usual dose of each may need to be reduced. Careful adjustment is necessary, depending on side effects and the degree of symptom control. Trihexyphenidyl dosage of 3-6mg daily in divided doses, is usually adequate.
Trihexyphenidyl may be taken before or after meals according to the way the patient reacts. If trihexyphenidyl tends to dry the mouth excessively, it may be better to take it before meals, unless it causes nausea. If taken after meals, induced thirst can be allayed by peppermint, chewing gum or water.
The size and frequency of dose of trihexyphenidyl needed to control extrapyramidal reactions to commonly employed tranquillisers, notably the phenothiazines, thioxanthenes, and butyrophenones must be determined empirically. The total daily dosage usually ranges between 5 and 15mg, although in some cases, these reactions have been controlled by as little as 1mg daily.
Satisfactory control may sometimes be more rapidly achieved by temporarily reducing the dosage of both drugs until the desired ataractic effect is retained without concomitant extrapyramidal reactions.
It is sometimes possible to maintain the patient on reduced trihexyphenidyl dosage after the reactions have remained under control for several days. Since these reactions may remain in remission for long periods after discontinuation of trihexyphenidyl therapy, such therapy should be of minimal duration and discontinued after symptoms have subsided for a reasonable period of time.
Patients over 65 years of age tend to be relatively more sensitive and require smaller amounts of the drug.
Not recommended.
Symptoms of overdose with antimuscarinic agents include flushing and dryness of the skin, dilated pupils, dry mouth and tongue, tachycardia, rapid respiration, hyperpyrexia, hypertension, nausea, vomiting. A rash may appear on the face or upper trunk. Symptoms of CNS stimulation include restlessness, confusion, hallucinations, paranoid and psychotic reactions, incoordination, delirium and occasionally convulsions. In severe overdose, CNS depression may occur with coma, circulatory and respiratory failure and death.
Treatment should always be supportive. An adequate airway should be maintained. Diazepam may be administered to control excitement and convulsions but the risk of central nervous system depression should be considered. Hypoxia and acidosis should be corrected. Antiarrhythmic drugs are not recommended if dysrhythmias occur.
Shelf life: 60 months.
Store below 25°C. Store in the original package in order to protect from moisture.
HDPE bottle with a child-resistant closure. Pack sizes of 28, 30, 56, 60, 84, 100 and 1000 tablets.
Not all pack sizes may be marketed.
No special requirements.
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