The most common adverse effects of valproic acid are digestive complaints like diarrhea, nausea, vomiting and indigestion; vision problems like seeing double or lazy eye; hormonal disturbances (increased testosterone production in females and menstrual irregularities), hair loss, memory problems, weight gain, infections, low platelet count (which can make one bleed more easily), dizziness, drowsiness, tremor and headache. Less common, yet serious side effects include liver damage, brittle bones (becomes far more common with long-term use), polycystic ovaries, movement disorders (which may be irreversible like tardive dyskinesia), psychiatric/neurologic disturbances like hallucinations, anxiety and confusion; swollen pancreas, low body temperature and potentially life-threatening blood abnormalities.
Extrapyramidal syndrome (including parkinsonism, may be reversible)
Brain problems due to high ammonia levels
Hypersensitivity reactions including multi-organ hypersensitivity syndrome
Eosinophilic pleural effusion
Bone fractures (reduced BMD with long-term use)
Other possible side effects
There is evidence that valproic acid may cause premature growth plate ossification in children and adolescents, resulting in short stature. Valproic acid can also cause mydriasis, a protracted dilation of the pupils
Valproate causes birth defects; exposure during pregnancy is associated with about three times as many major anomalies as usual, mainly spina bifida and, more rarely, with several other defects, possibly including a "valproate syndrome". Characteristics of this valproate syndrome include facial features that tend to evolve with age, including trigonocephaly, tall forehead with bifrontal narrowing, epicanthic folds, medial deficiency of eyebrows, flat nasal bridge, broad nasal root, anteverted nares, shallow philtrum, long upper lip and thin vermillion borders, thick lower lip and small downturned mouth.
Women who intend to become pregnant should switch to a different drug if possible. Women who become pregnant while taking valproate should be warned that it causes birth defects and cognitive impairment in the newborn, especially at high doses (although vaproate is sometimes the only drug that can control seizures, and seizures in pregnancy could have even worse consequences.) They should take high-dose folic acid and be offered antenatal screening (alpha-fetoprotein and second-trimester ultrasound scans), although screening and scans do not find all birth defects.
Valproate can cause neural tube defects. Folic acid supplements may reduce the risk of birth defects, however. A recent study showed children of mothers taking valproate during pregnancy are at risk for significantly lower IQs. Maternal valproate use during pregnancy has been associated with a significantly higher risk of autism in the offspring. Exposure of the human embryo to valproic acid is associated with risk of autism, and it is possible to duplicate features characteristic of autism by exposing rat embryos to valproic acid at the time of neural tube closure. Valproate exposure on embryonic day 11.5 led to significant local recurrent connectivity in the juvenile ratneocortex, consistent with the underconnectivity theory of autism. A 2009 study found that the 3 year old children of pregnant women taking valproate had an IQ nine points lower than that of a well-matched control group. However, further research in older children and adults is needed.
Valproate inhibits CYP2C9, glucuronyl transferase, and epoxide hydrolase and is highly protein bound and hence may interact with drugs that are substrates for any of these enzymes or are highly protein bound themselves. It may also potentiate the CNS depressant effects of alcohol. It should not be given in conjunction with other antiepileptics due to the potential for reduced clearance of other antiepileptics (including carbamazepine, lamotrigine, phenytoin and phenobarbitone) and itself. It may also interact with:
Anticoagulants, due to its ability to prolong the bleeding time.
Benzodiazepines; may potentiate CNS depression and there are possible pharmacokinetic interactions.
Ethosuximide; potential for ethosuximide toxicity.
Primidone; may reduce pyrimidone's clearance leading to toxicity.
Zidovudine; may raise its (zidovudine's) serum concentration and lead to toxicity.
Aspirin; may displace valproate from plasma proteins, leading to increased plasma concentrations. Also interferes with valproate's metabolism.
Felbalmate; may increase plasma concentrations of valproate.
Mefloquine; potential for increased valproate metabolism combined with the direct epileptogenic effects of mefloquine.
Cimetidine; inhibits valproate's metabolism in the liver, hence leading to reduced plasma concentrations of valproate.
Erythromycin; inhibits valproate's metabolism in the liver, hence leading to increased plasma concentrations of valproate.
Carbapenem antibiotics; reduces valproate levels, potentially leading to seizures.
Overdose and toxicity
Excessive amounts of valproic acid can result in tremor, stupor, respiratory depression, coma, metabolic acidosis, and death. Overdosage in children is usually of an accidental nature, whereas with adults it is more likely to be an intentional act. In general, serum or plasma valproic acid concentrations are in a range of 20–100 mg/l during controlled therapy, but may reach 150–1500 mg/l following acute poisoning. Monitoring of the serum level is often accomplished using commercial immunoassay techniques, although some laboratories employ gas or liquid chromatography. In contrast to other antiepileptic drugs, at present there is little evidence favorable evidence for salivary therapeutic drug monitoring. Salivary levels of valproic acid correlate poorly with serum levels, at least in part due to valproate's weak acid property (pKa of 4.9).
Although the mechanism of action of valproate is not fully understood, it has recently been shown to protect against a seizure-induced reduction in phosphatidylinositol (3,4,5)-trisphosphate (PIP3) as a potential therapeutic mechanism. In addition, its anticonvulsant effect has been attributed to the blockade of voltage-dependent sodium channels and increased brain levels of gamma-aminobutyric acid (GABA). The GABAergic effect is also believed to contribute towards the anti-manic properties of valproate. In animals, sodium valproate raises cerebral and cerebellar levels of the inhibitory synaptic transmitter, GABA, possibly by inhibiting GABA degradative enzymes, such as GABA transaminase, succinate-semialdehyde dehydrogenase and by inhibiting the re-uptake of GABA by neuronal cells. It also possesses histone deacetylase-inhibiting effects. The inhibition of histone deacetylase, by promoting more transcriptionally active chromatin structures, likely presents the epigenetic mechanism for regulation of many of the neuroprotective effects attributed to valproic acid. Intermediate molecules mediating these effects include VEGF, BDNF, and GDNF. 
Valproic acid was first synthesized in 1882 by B.S. Burton as an analogue of valeric acid, found naturally in valerian. Valproic acid is a carboxylic acid, a clear liquid at room temperature. For many decades, its only use was in laboratories as a "metabolically inert" solvent for organic compounds. In 1962, the French researcher Pierre Eymard serendipitously discovered the anticonvulsant properties of valproic acid while using it as a vehicle for a number of other compounds that were being screened for antiseizure activity. He found it prevented pentylenetetrazol-induced convulsions in laboratory rats. It was approved as an antiepileptic drug in 1967 in France and has become the most widely prescribed antiepileptic drug worldwide. Valproic acid has also been used for migraine prophylaxis and bipolar disorder.
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