Injections are vital to delivering treatment for many different causes and conditions. In almost all but a few cases, they are perfectly safe and cause only minor discomfort.

There are times, however, when a person may experience an adverse response, often in the form of an infection or allergy. Some may be minor and easily treated. Others may be far more serious and lead to a potentially deadly, all-body reaction (such as anaphylaxis or sepsis).

The symptoms can vary depending on whether the shot was delivered subcutaneously (under the skin), intravenously (into a vein) or intramuscular (into a muscle).

Call your doctor immediately if you experience any of the following symptoms:

High Fever

Sick man taking temperature with digital thermometer — Tom Merton/Getty Images

If ever you have a fever higher than 101 F following an injection, call your doctor or visit the nearest emergency room. The fever may be the result of an infection caused by needle contamination or an allergic reaction to the medication itself. Both are considered serious.

By and large, allergies tend to happen quickly while an infection may take one to 10 days before symptoms appear.

While many infections occur as a result of a self-administered injection, they can also happen at the doctor's office or in the hospital if aseptic techniques are not adhered to.

Extreme Pain at the Injection Site

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While most people dislike the idea of a shot, it is usually quickly over and causes little pain. However, if the pain persists or worsens, you should call a doctor and have looked at.

While it not uncommon to have localized swelling or redness for a day or two following an injection (or even longer for certain types of intramuscular shots), those that are deeply felt, tender to the touch, or accompanied by fever, body aches, or creeping discoloration should never be ignored.

In some cases, the pain may be extreme but not particularly dangerous (such as when an intramuscular injection accidentally hits the sciatic nerve). But, at other times, it may be due to an infection that might only get worse if left untreated.

Swelling or Hardness Under the Skin

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While swelling and minor bruising can happen after a shot, they usually get better within a day or so. If swelling and discoloration persist, it may be the sign of an infection.

Abnormal swelling that feels soft, mushy, and painful may the indication of a developing an abscess. An abscess is a walled-off collection of pus. It is often warm to the touch and may be accompanied by the enlargement of nearby lymph nodes.

Abscesses should never be squeezed. If the abscess is not properly drained and is allowed burst under the skin, the infection can spread through the bloodstream and cause a potentially life-threatening blood infection known as sepsis.

While a little drainage following an injection may be normal (caused by medication leaking out of the needle track), any discolored or abnormal discharge should be looked at immediately.

If the bump is small and you're not sure if it's an abscess, take a pen and draw a circle along the border. If it starts to expand beyond the border or fails to go away in several hours, call a doctor and have it looked at as soon as possible.

A Sudden, All-Body Reaction

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The most serious reaction following an injection is an all-body, allergic response known as anaphylaxis. This can occur if the body reacts adversely to the injected medication, causing a cascade of severe and potentially life-threatening symptoms.

Anaphylaxis develops very quickly and needs to be treated immediately with a shot of epinephrine (adrenaline).

The first signs of anaphylaxis may be similar to those for an allergy, including a runny nose and congestion (rhinitis) and an itchy skin rash. However, within 30 minutes or so, more serious symptoms can develop, including:

Coughing, wheezing, and shortness of breath
Chest tightness
Hives
Dizziness or fainting
Rapid or irregular heartbeat
Weak pulse
Facial swelling
Swollen or itchy lips or tongue
Difficulty swallowing
A blue-ish tinge to the lips, fingers, or toes (cyanosis)
Pale, clammy skin
Nausea, vomiting, or diarrhea

People who have anaphylaxis often report having a feeling of impending doom and panic. If left untreated, anaphylaxis can lead to shock, coma, or even death.