Antipsychotic drugs can be life-saving for many men with bipolar, schizophrenic, manic and other disorders. Unfortunately, they can also negatively impact testosterone levels and sexual function in a variety of ways. This should not be surprising as psychotropic drugs of any sort are very powerful medications and hormones are often impacted.
Antipsychotic medications are a very involved subject and one can’t help but wonder how well understood they are by even the experts. However, there are several key categories of these drugs, which helps in understanding their potential impact on testosterone. One category are the “typical antipsychotics,” which have a relatively common side effect of Parkinson-like symtpoms, including tremors and rigidity since they block dopamine receptors in the brain. (NOTE: These symptoms can become permanent.)
The atypical, second generation antipsychotics do not have the advantage of avoiding Parkinson-like symptoms, but, of course, have a separate set of nasty side effects that pateints must be aware of and watch out for, including diabetes, movement disorders and stroke just for starters.
It is the typical antipsychotics that were known for potentially lowering testosterone. The reason is that these medications can lead to hyperprolactinemia, or elevated prolactin levels. As you hopefully know from my link on Prolactin and Prolactinomas, overly high prolactin levels can hammer your testosterone.
One study showed that in females testosterone was strongly affected by these medications, but in males not as much.  Another study on male schizophrenics found that switching from the typicals to the atypicals did, indeed, lower prolactin levels. However, testosterone levels remained largely unchanged.  In other words, these typical medications likely lower testosterone a little, but not enough to make a significant difference.
CAUTION: This is informational only. Do not go off or change any medication without consulting your physician.
Or is that really the whole story? Actually, some of these drugs have been found to lower testosterone. For example, the second generation antipsychotic Risperdal (risperidone) was shown to lower testosterone in rats.  Interestingly enough, it not affect leutinizing hormone, but still lowered testosterone through some other mechanism. In addition, the drug carbamazepine (Carbatrol, Tegretol, Equetro, Epitol) used for bipolar and manic disorders has also been shown to increase clearance of testosterone and thus likely decrease plasma testosterone levels. 
Regardless of short term affects on testosterone according to the studies, it is unlikely that these drugs will do anything but lower testosterone in the long term. Consider these risk factors for lower testosterone that are generally negatively impaced:
1. Dopamine. Decreased levels of dopamine lead to sexual dysfunction and decreased sexual desire. This will likely lead to less sexual activity and less sexual activity will tend to decrease testosterone.
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2. Diabetes and Metabolic Disorders. A significant risk factor for erectile dysfunction is type II (adult onset) diabetes. This is no surprise as diabetes is incredibly hard on the body and ages many tissues at an accelerated rate. The atypical antipsychotics are known to increase risk for diabetes.  Follow up research has verified this affect across a wide variety of the atypicals.  Blood sugar disorders will do nothing but lower testosterone, potentially total and free testosterone with time, due to the increased oxidative and inflammatory load placed on the body.
3. Prolactin. Elevated prolactin levels are a known libido killer in and of itself.  Again, this will tend to lead to decreased sex and sexual interest, which will probably lower testosterone long term (and even short term as mentioned above).
Therefore, long term studies my show other decreases in testosterone levels as well.
1) Journal of Clinical Psychopharmacology, 2002, 22:109-114, “The effects of anti-psychotic induced hyperprolactinaemia on the hypothalamic-pituitary-gonadal axis”
2) Neuroendocrinology Letters, Nos.1/2, Feb-Apr Vol.25, 2004, “Impact of a switch from typical to atypical antipsychotic drugs on quality of life and gonadal hormones in male patients with schizophrenia”
3) J Psychopharmacol, Jun 2007, 21(4):428-434, “The effect of chronic antipsychotic treatment on sexual behaviour, hormones and organ size in the male rat&http://priory.com/psych/sexdys.htm
5) Am J Psychiatry, Apr 2002, 159:561-566, “Association of Diabetes Mellitus With Use of Atypical Neuroleptics in the Treatment of Schizophrenia”
6) Psychopharmacol Bull, 2009; 42(1):1-21, “Atypical Antipsychotic Drugs and Diabetes Mellitus in the US Food and Drug Administration Adverse Event Database: A Systematic Bayesian Signal Detection Analysis”
7) Journal of Endocrinology, 2003, 179:357 365, “Effects of acute prolactin manipulation on sexual drive and function in males”