The short answer

Recovery after PRP for knee osteoarthritis is usually gradual rather than immediate. Patient guidance from Washington University Orthopedics and Mass General Brigham says the first few days can be an inflammatory phase, with a temporary pain flare or local discomfort lasting up to about a week, while benefit often becomes noticeable over roughly 6 to 8 weeks.

Repeat sessions are common, but not standardised. In a randomised knee OA study of 120 patients followed for 52 weeks, three injections given 1 week apart improved pain, stiffness and function more than a single injection, and were not significantly different from five injections. Because PRP preparation and dosing vary across studies and clinics, there is no single universal schedule. A 2024 safety review found infection was the most commonly reported adverse event in the literature, so sudden severe pain in one joint, marked swelling, skin colour change, fever, or feeling hot, cold or shivery need urgent review under NHS septic arthritis guidance.

How recovery usually unfolds

Day 1 is often too early to judge whether PRP has helped a knee.

• First few days: Washington University Orthopedics describes this as an inflammatory phase. In that early window, a temporary increase in ache, stiffness or mild swelling around the knee can happen even when the injection is following an expected course.
• By the end of week 1: Mass General Brigham notes that local discomfort can last up to about a week. That sort of soreness on its own is not the same as a treatment failure, and it is different from a sudden major deterioration.
• Around weeks 6 to 8: both hospital-style patient guidance and clinical practice point to a slower build. Washington University says benefit may take about 6 to 8 weeks to become noticeable, and Mass General Brigham similarly frames PRP as something other than short-term pain management.

The pace is not identical for every knee. Mass General Brigham notes that PRP is often combined with physical therapy, so progress may depend partly on the rehabilitation plan and activity levels after the injection. Published knee OA research also suggests severity may matter: in a 24-month study, maximal pain relief appeared later in KL 3-4 knees than in KL 1-2 knees within a treatment series.

Why clinics recommend more than one injection

Many clinics plan PRP for knee osteoarthritis as a short course rather than a one-off injection. The clearest plain-language takeaway from the current evidence is that three weekly injections sit on the strongest randomised footing, while a higher number may be reasonable in some more advanced knees rather than as a default for everyone.

In a 52-week randomised study of 120 patients with grade I to III knee OA, three or five injections given 1 week apart improved pain, stiffness and function more than one injection. Crucially, three injections were not clearly worse than five across that follow-up, which helps explain why some clinics favour a three-session plan when balancing treatment burden with expected benefit.

A longer 24-month observational study points to a slightly different pattern once OA is more severe. In that series, peak pain relief appeared after about the 4th injection in KL 1 to 2 knees and after the 5th injection in KL 3 to 4 knees. That does not prove one perfect schedule, but it does support a more specific consultation question: why this number of injections for this Kellgren-Lawrence grade, and what evidence is the clinic using for that choice?

When symptoms are not a normal flare

The pattern matters more than the mere presence of pain. A brief ache after an injection can fit the usual first-week settling period, but a knee that becomes suddenly much more painful, swollen or hot is a different picture and should not be dismissed as a routine flare.

A 2024 review of adverse events linked to PRP reported infections, inflammation, allergic reactions and nodules, with infection the most commonly reported adverse event in the literature it surveyed. That does not make infection the expected outcome after knee PRP, but it does make it the complication clinicians need to keep in mind when symptoms are escalating rather than easing.

The NHS advises urgent assessment for possible septic arthritis when there is sudden severe pain in one joint, marked swelling, a change in skin colour around the joint, fever, or feeling hot, cold or shivery. In practice, a knee that is becoming dramatically more painful or swollen after PRP usually warrants prompt contact with the treating team or urgent care rather than watchful waiting.

Questions to ask before you agree

For knee OA, the biggest differences between PRP clinics often sit in the treatment plan rather than the injection itself: a 52-week trial and a 24-month follow-up study both used repeat treatment, but not the same schedule for every knee. Useful comparison questions include:

• “How many injections are you recommending, how far apart, and why that number for my Kellgren–Lawrence grade?” In published knee OA studies, 3 weekly injections performed better than 1, while a 24-month series suggested that some more advanced knees may peak after 4 or 5 injections.
• “How is the PRP prepared and kept sterile from blood collection to processing to injection?” A 2024 safety review highlighted contamination prevention at each step.
• “Who reviews progress, when is it reassessed, and what happens if the first injection or first course does not help?” A clear follow-up plan matters as much as the procedure.
• “Is physiotherapy part of the plan, and what activity limits apply in the first few days?” Hospital guidance commonly places PRP within a wider rehab pathway rather than as a quick fix.
• “Are there any reasons PRP may be unsuitable for me?” Individual contraindications and exclusion criteria still need to be checked directly with the treating clinician.

Choosing a PRP specialist

For knee OA, the more useful final test is not the clinic name but whether the consultation covers the first week, the 6–8 week recovery horizon, and NHS red flags if the knee worsens rather than settles.

• A strong consultation sets a realistic timeline: a short inflammatory flare and local discomfort can happen in the first few days, and improvement may take 6–8 weeks rather than appearing straight away.
• It explains the treatment plan without blanket promises. Published knee-OA studies have used different PRP schedules, so a credible clinician should say why a single injection or a series is being proposed.
• It includes a structured aftercare plan, with physiotherapy or rehabilitation, follow-up arrangements, and clear advice to seek urgent review for symptoms such as sudden severe single-joint pain, marked swelling, skin colour change or fever.

PRP may be a reasonable option for some people with knee OA, but it is not a one-size-fits-all answer. For comparing options across the UK, Search MSK lists specialists who offer PRP and knee osteoarthritis care, with filters by region and specialty.