Before taking virtually any medication, you as the patient must ask yourself: will there be side effects, and if so, what will they be? Many side effects are minor, such as nausea or fatigue, but some can be serious or even life-threatening. Read on to discover nine of the most frightening medication-induced side effects. Although the risks of these side effects are typically very low, they're real possibilities acknowledged by the Food and Drug Administration (FDA) and/or the manufacturer of the drug:

1. Blindness

In response to reports of sudden blindness in men taking erectile dysfunction medications, in 2005, the FDA mandated updated labeling for drugs containing sildenafil citrate, tadalafil, or vardenafil as their active ingredient. The blindness risk was reported in June of 2005 in the Journal of Neuro-Ophthalmology, which reported seven cases of unusual sudden blindness within 36 hours of taking the drug. The FDA received 38 additional reports of men experiencing impaired vision after taking the drug. These cases were related to reduced blood flow to the eyes in patients who were also suffering from high blood pressure and diabetes, according to the FDA. It is most common in older people. Patients who experience sudden or decreased vision loss in one or both eyes should stop taking the medications and to contact their doctor immediately, the FDA advises.

2. Black Tongue

Surprisingly, this side effect is not as scary as it looks. A compound of the element bismuth called bismuth subsalicylate has properties that have antidiarrheal, antibacterial, and antacid effects in the digestive tract, and it is commonly used in antidiarrheal over-the-counter medications. When it combines with sulfur in human saliva and in the gastrointestinal tract, it creates a black-colored substance called bismuth sulfide. This is what creates this deceptively frightening black tongue or even darkened stool. The discoloration can linger for a few days, but it's fortunately harmless.

3. Addictive Behaviors

Parkinson's disease is a degenerative disorder of the central nervous system that usually impairs the patient's movement and speech, among other things. Neurologists have reported seeing drastic changes in the behavior of patients receiving treatment for Parkinson's disease, with some developing sudden addictions, such as gambling problems, elevated sexual interest, or compulsive spending and eating habits. Recently, the largest study ever conducted on the trend has found that more than 13 percent of patients taking dopamine agonists (which are used as treatment for Parkinson's) suffer from at least one of four serious behavioral addictions.

4. Brain Disease

Multiple sclerosis (MS) is a chronic disease that attacks the central nervous system. A drug with the active ingredient natalizumab was considered the most promising MS drug in years. However, it appears to sometimes cause another disease called progressive multifocal leukoencephalopathy (PML), which resembles a speedier and deadlier form of MS. PML is caused when the JC virus, which is dormant in most people's bodies, takes advantage of a weakened immune system. Patients interested in this drug must go through a unique, strict compliance plan and discuss the benefits and risks with their doctors before it is prescribed.

5. Burning from the Inside

Stevens-Johnson Syndrome (SJS) is a potentially deadly skin disease, usually caused by a drug reaction. The frightening allergic reaction has been compared to being burned from the inside and can be caused by a plethora of medicines, including painkillers, epilepsy drugs, and antibiotics. The sufferer's body first develops a rash, which can lead to blisters—even inside the mouth and lungs. In severe cases, if patients are not treated, they can bleed to death, go blind, or have their pores scar shut. The FDA pulled one medication from the market due, in part, to an increased risk of SJS.

6. Strange Sleep Behaviors

The FDA, in March 2007, requested that all sedative-hypnotic drug products (that is, sleeping pills) include information about the possible risk of sleep behaviors, including driving while sleeping. Worldwide, there have been reports of adverse reactions to prescription medications used for the short-term treatment of insomnia. These reactions include a woman who did not know why she had gained more than 44 pounds until she found out that she was binge eating from the fridge during her sleep. Another patient woke up holding a paintbrush, only to discover that she'd painted the front door in her sleep. There were also reports given that suggest patients may have driven, made phone calls, or had sexual intercourse--all while sleeping--and with no recollection of having participated in these acts.

7. Suicide

One of the worst risks of clinical depression is suicide. So, the discovery that the drugs frequently used to treat the disease may heighten the risk of suicide in adolescents, was quite devastating. All of the drugs now contain warnings stating that short-term studies and analyses demonstrate an increased risk of suicidal thoughts, related behaviors, and adverse events—advising parents and doctors to exercise caution.

8. Destroyed Muscles

Heart attack is a leading killer in the United States, but cholesterol pills are life-saving breakthroughs that can reduce the risk of heart attacks by more than a third. In rare cases, however, they may cause the destruction of muscles, which can harm the liver and even lead to death. This is a condition, known as rhabdomyolysis, which forced one cholesterol pill to be removed from the market. In March 2005, the labeling on drugs containing the active ingredient rosuvastatin calcium was revised to issue a warning about the possibly increased risk of rhabdomyolysis while taking the medication.