# Cat After an Injection or Shot: Lumps, Soreness and Reactions A cat after injection often has a small lump, a tender leg, or a day of sleepiness, and these reactions usually fade on their own within a few days. The problem is telling a harmless bump from a serious one. Most injection-site reactions are mild, but swelling that keeps growing, trouble breathing, or a cat that collapses needs a veterinarian immediately. This article is educational and is not a substitute for veterinary diagnosis or treatment. ## Quick Answer - A soft lump under the skin at the injection site is common and often goes away within a few days to a few weeks. - Mild sleepiness, a slightly reduced appetite, and a mildly raised body temperature are the most commonly reported reactions and often mean the immune system is responding well [1]. - Hives, facial swelling, vomiting, or collapse within minutes to hours of a shot are emergencies, and anaphylactic shock can be fatal without treatment [2]. - Any lump that is still growing one month after the injection, is larger than about 2 cm, or is still present three months later should be examined by a veterinarian. - Never give your cat human pain relievers. Many are toxic to cats, and your veterinarian will calculate any safe medication dose from your cat's weight and health. ## What Happens After a Shot and Why It Is Done Injections are one of the most common procedures in feline medicine. Your cat may receive a vaccine, an antibiotic, a steroid, a vitamin, or a fluid bolus under the skin. Each of these involves a needle passing through skin and muscle, and each can leave a mark. Vaccines work by priming the immune system. They expose T cells and B cells to antigens from an infectious organism so the body can respond faster and more strongly the next time it meets that germ [3]. That priming process is exactly why mild reactions happen. The immune system is doing its job. Antibiotic injections are given when a cat needs fast drug levels, cannot take pills, or is vomiting. Some long-acting antibiotics are designed to sit in the tissue and release slowly, which can leave a firmer, longer-lasting lump than a vaccine. Sometimes a cat is injected after a bite wound. If your cat was bitten by another cat, the injection is usually an antibiotic plus pain relief, and the wound itself may swell and drain for several days. A cat bite after injection can look worse before it looks better, so keep the area clean and watch for spreading redness or a bad smell. ## How to Prepare Before the Appointment You do not need to do much, but a few steps help. - Write down any reaction your cat had to a previous shot, including the date and what you saw. - Tell your veterinarian about all medications, supplements, and any illness in the past two weeks. - Ask which vaccines and injections are planned and where they will be given. - Ask whether the injections can be spread across visits. The risk of vaccine-associated adverse events increases with the number of vaccines given per visit, so minimizing the number of components per visit is reasonable [1]. - Bring a carrier with a secure latch and a towel inside. A cat that feels unwell after a shot may try to bolt. If your cat has had a severe reaction before, say so clearly at check-in. Your veterinarian may pre-treat with an antihistamine or steroid, or choose a different product. ## What Happens During the Injection Most injections take seconds. Your veterinarian or technician will part the fur, clean the skin, and slide the needle under the skin (subcutaneously) or into a muscle. Your cat may flinch, vocalize, or try to pull away. That is normal. Vaccines have historically been given in the scruff, the loose skin over the shoulders, because the space is easy to access [4]. In recent years, some veterinarians have moved certain vaccines to a limb or the tail so that if a sarcoma ever develops, the limb can be amputated with clean margins. Research on feline leukemia vaccination has compared antibody responses when the vaccine is given in the scruff, the left hindlimb, or the tail [4]. Ask your veterinarian where they plan to inject and why. After the needle is out, pressure is applied for a few seconds. You may see a tiny drop of blood. That is fine. ## Recovery Timeline and Home Care Most cats are back to normal within a day. Some are quiet for 24 to 48 hours. Use this table as a general guide, and remember that every cat is different. | Time after injection | What you may see | What to do | |---|---|---| | First 30 minutes | Mild soreness, a small drop of blood, normal behavior | Keep your cat calm and watch for hives or facial swelling | | 1 to 6 hours | Sleepiness, reduced appetite, mild tenderness at the site | Offer water and a small meal, let your cat rest | | 6 to 24 hours | A soft lump the size of a pea or larger, mild fever | Check the site once, do not squeeze or massage it | | 1 to 3 days | Lump may feel slightly firmer, appetite returns | Keep the area clean, limit rough play | | 3 days to 3 weeks | Lump shrinks and disappears in most cats | Check the size weekly and note any change | | 1 to 3 months | A lump that is unchanged, growing, or still present | Have your veterinarian examine it | Home care is simple. Do not rub the injection site. Do not apply heat or ice unless your veterinarian tells you to. Do not give any medication, including over-the-counter pain relievers, without asking first. Many human pain drugs are toxic to cats. If your cat is sleepy, let them sleep. If your cat refuses food for more than 24 hours, call your veterinarian. If your cat is vomiting, has diarrhea, or seems to be in pain, call sooner. For a deeper look at how to tell a mild reaction from a serious one, see [Vaccine Reactions in Dogs and Cats: Anaphylaxis vs Mild Lethargy Triage](/knowledge/veterinary-medicine/preventive-care/vaccine-reactions-in-dogs-and-cats-anaphylaxis-vs-mild-lethargy-triage). ## Possible Complications and Warning Signs ### Mild Local Reactions A small, soft lump at the injection site is the most common local reaction. It may feel like a pea or a small grape. It should not be hot, red, or painful to the touch. Most shrink within a few weeks. ### Mild Systemic Reactions Sleepiness, a mildly reduced general condition, loss of appetite, and a mildly increased body temperature are the most commonly observed vaccine-associated adverse events in cats [1]. These signs often indicate a good immune response and usually resolve without treatment [1]. ### Allergic Reactions and Anaphylaxis Anaphylactic shock is a rare, life-threatening allergic reaction to an injection, food, or an insect sting. Signs usually start within seconds to minutes of exposure and include severe breathing difficulty, sudden vomiting or diarrhea, heavy drooling, excitement, incoordination, shock, seizures, and collapse [2]. The gums look very pale, the limbs feel cold, and the pulse is weak even though the heart rate is fast [2]. Facial swelling and itching around the face and head may also occur [2]. Hives are the least severe type of anaphylactic reaction. They appear as small bumps on the skin, often within 20 minutes of exposure, and the hair may stand up over them [2]. Swelling of the lips, nose, or around the eyes is more serious and can be severe enough that the cat cannot open its eyes [2]. ### Feline Injection-Site Sarcoma Feline injection-site sarcoma is a rare but serious tumor that can develop after a vaccine or after injection of other substances [5]. The most widely accepted explanation is that long-term inflammation at the injection site triggers malignant transformation, though the exact process is not fully understood [5]. These tumors are aggressive and can require major surgery. One case report described a cat that needed chest wall resection and forequarter amputation for a sarcoma that had infiltrated the soft tissues of the chest [6]. The good news is that this is uncommon. A 2026 systematic review and meta-analysis was conducted specifically to estimate how often feline injection-site sarcoma occurs after vaccination, because published estimates vary widely and create uncertainty for owners and veterinarians [7]. If you want to understand the odds in plain terms, ask your veterinarian to walk you through the current estimate. ### When a Lump Needs a Closer Look Any lump that is still growing one month after the injection, is larger than 2 cm, or is still present three months later should be checked. Your veterinarian may measure it, take a fine needle sample, or recommend surgical removal. Early removal gives the best chance of a clean margin. For a broader guide to lumps on cats, including ones that are not related to injections, see [Cat Skin Bumps and Lumps: When to See a Vet and When to Monitor](/knowledge/veterinary-medicine/vet-visit-decisions/cat-skin-bumps-lumps). ## Questions to Ask Your Veterinarian - Which injections are being given today, and where on the body will each one go? - What reaction should I expect, and how long should it last? - What signs mean I should call you right away? - Is there a way to reduce the number of injections per visit? - Can we check antibody levels instead of revaccinating every year? Antibody testing can be used to assess whether revaccination is needed, at least for panleukopenia in adult cats [1]. - If a lump appears, when should I bring my cat back for a recheck? ## Limitations and When to Contact a Veterinarian No article can tell you what is happening under your cat's skin. Your veterinarian can feel the lump, measure it, and decide whether testing is needed. Individual cases always need a hands-on exam. Call your veterinarian the same day if: - Your cat has a lump that is hot, red, or painful - Your cat has not eaten in 24 hours - Your cat is vomiting or has diarrhea that lasts more than a few hours - Your cat seems painful when walking or will not let you touch the leg that was injected - The lump is growing or is larger than 2 cm Go to an emergency clinic now if: - Your cat is having trouble breathing or is breathing fast with the mouth open - Your cat's face, lips, or eyes are swelling - Your cat has hives over the body - Your cat is vomiting repeatedly, drooling heavily, or having seizures - Your cat's gums are pale, the limbs feel cold, or your cat collapses - Your cat cannot stand or seems unaware of surroundings These are signs of anaphylaxis, and they can progress within minutes [2]. Do not wait to see if it passes. ## Frequently Asked Questions ### How long does a lump last after a shot? Most lumps shrink within a few weeks. A lump that is still growing at one month, is larger than 2 cm, or is still present at three months should be examined. Your veterinarian can measure it and decide whether a sample is needed. ### Is it normal for my cat to sleep a lot after a vaccine? Yes, mild sleepiness is one of the most commonly reported reactions and often signals a good immune response [1]. It usually clears within a day or two. If your cat is difficult to rouse, is breathing oddly, or will not eat, call your veterinarian. ### Can an antibiotic injection cause a lump? Yes. Long-acting antibiotic injections can leave a firmer lump that lasts longer than a vaccine lump. Watch for heat, redness, or pain, and have it checked if it is still there after a few weeks. ### What should I do if my cat was bitten and then got a shot? Keep the bite wound clean and watch for swelling, drainage, or a bad smell. A cat bite after injection can look worse before it improves. Call your veterinarian if the swelling spreads, your cat stops eating, or your cat seems painful. ### Can I give my cat pain medicine at home? No. Many human pain relievers are toxic to cats, and the wrong dose can be fatal. Your veterinarian will calculate a safe dose from your cat's weight and health if pain medication is needed. Never give anything without asking first. If your cat has a lump that worries you, or if you are unsure whether a reaction is mild or serious, call your veterinarian. A quick exam is the fastest way to know. ## Related Articles - [Vaccine Reactions in Dogs and Cats: Anaphylaxis vs Mild Lethargy Triage](/knowledge/veterinary-medicine/preventive-care/vaccine-reactions-in-dogs-and-cats-anaphylaxis-vs-mild-lethargy-triage) - [Cat Skin Bumps and Lumps: When to See a Vet and When to Monitor](/knowledge/veterinary-medicine/vet-visit-decisions/cat-skin-bumps-lumps) - [Cat Spraying After Neutering: Why It Happens and What to Do](/knowledge/veterinary-medicine/anxiety-and-behavior-support/cat-spraying-after-neutering) - [Aortic Thromboembolism FATE in Cats: Saddle Thrombus Pain and Prognosis](/knowledge/veterinary-medicine/internal-medicine/aortic-thromboembolism-fate-in-cats-saddle-thrombus-pain-and-prognosis) - [Cat Skin Bumps and Lumps: What the Vet Will Do and How Much It Costs](/knowledge/veterinary-medicine/vet-visit-decisions/cat-skin-bumps-lumps-vet-cost) - [Cat Vaccines Core](/knowledge/veterinary-medicine/preventive-care/cat-vaccines-core) ## References 1. [Vaccine-associated adverse events in cats](https://pubmed.ncbi.nlm.nih.gov/39965621/). Tierarztliche Praxis. Ausgabe K, Kleintiere/Heimtiere, 2025. 2. [Disorders Involving Anaphylactic Reactions (Type I Reactions, Atopy) in Cats - Cat Owners](https://www.merckvetmanual.com/cat-owners/immune-disorders-of-cats/disorders-involving-anaphylactic-reactions-type-i-reactions-atopy-in-cats). Merck Veterinary Manual. 3. [Immune System Responses in Cats - Cat Owners](https://www.merckvetmanual.com/cat-owners/immune-disorders-of-cats/immune-system-responses-in-cats). Merck Veterinary Manual. 4. [Antibody responses to feline leukaemia vaccination: exploring the effect of sex, boosting schedule, site of administration and vaccine type](https://pubmed.ncbi.nlm.nih.gov/40905267/). Journal of feline medicine and surgery, 2025. 5. [Feline Injection-Site Sarcoma and Other Adverse Reactions to Vaccination in Cats](https://pubmed.ncbi.nlm.nih.gov/37632050/). Viruses, 2023. 6. [Ventilatory failure in a cat following radical chest wall resection for feline injection site sarcoma](https://pubmed.ncbi.nlm.nih.gov/34350025/). JFMS open reports, 2021. 7. [The incidence of vaccine-associated sarcomas in cats: a systematic review and meta-analysis](https://pubmed.ncbi.nlm.nih.gov/42532508/). The Journal of small animal practice, 2026. ## Further Reading - [Disorders Involving Cell-mediated Immunity (Type IV Reactions) in Cats - Cat Owners (Merck Veterinary Manual)](https://www.merckvetmanual.com/cat-owners/immune-disorders-of-cats/disorders-involving-cell-mediated-immunity-type-iv-reactions-in-cats) - [American Veterinary Medical Association: First Aid Tips for Pet Owners](https://www.avma.org/resources-tools/pet-owners/emergencycare/first-aid-tips-pet-owners) - [Merck Veterinary Manual: Cat Owners](https://www.merckvetmanual.com/cat-owners)