severe cramping in his legs, from the knees down; woke up for about 50 minutes in the worst pain of his entire life; When he looked at the muscles, they were rock solid, hard; weakness in legs; excruciating pain in both extremities below the knees; I noticed weakness in both lower extremities; I was unable to rise from a sitting position in a chair; I recieved a booster shot; This is a spontaneous report received from contactable reporter(s) (Physician). The reporter is the patient.  A 78 year-old male patient received bnt162b2 (BNT162B2), administered in arm left, administration date 06Dec2021 (Lot number: 33130BA, Expiration Date: 31May2022) at the age of 78 years as dose 3 (booster), single for covid-19 immunisation. Relevant medical history included: "impairment of recent memory and mild impairment/memory loss" (unspecified if ongoing), notes: treating for some impairment of recent memory and mild impairment.; "Restless leg" (unspecified if ongoing); "Bipolar" (unspecified if ongoing); "severe spinal stenosis" (unspecified if ongoing); "Prostate cancer" (unspecified if ongoing); "known allergies: penicillin" (unspecified if ongoing); "known allergies: sulfa" (unspecified if ongoing). Concomitant medication(s) included: ASPIRINE (ongoing); MEMANTINE taken for memory impairment (ongoing); LITHIUM; PRAMIPEXOLE. Vaccination history included: Bnt162b2 (dose 1, Lot number: EN5318, Expiration date: 31May2021, Prev dose vaccine location: Left leg), administration date: 26Jan2021, when the patient was 78 years old, for COVID-19 Immunization; Bnt162b2 (Dose 2, Lot number: EN6201, Expiration date: 31May2021, Prev dose vaccine location: Left arm), administration date: 19Feb2021, when the patient was 78 years old, for COVID-19 Immunization. The following information was reported: IMMUNISATION (non-serious) with onset 06Dec2021, outcome "unknown", described as "I recieved a booster shot"; MUSCLE SPASMS (non-serious) with onset 07Dec2021 08:00, outcome "recovered" (07Dec2021 08:00), described as "severe cramping in his legs, from the knees down"; PAIN (non-serious) with onset 07Dec2021 08:00, outcome "recovered" (07Dec2021 08:00), described as "woke up for about 50 minutes in the worst pain of his entire life"; MUSCLE DISORDER (non-serious) with onset 07Dec2021 08:00, outcome "recovered" (07Dec2021 08:00), described as "When he looked at the muscles, they were rock solid, hard"; MUSCULAR WEAKNESS (non-serious) with onset 07Dec2021 08:00, outcome "recovered" (07Dec2021 08:00), described as "weakness in legs"; ARTHRALGIA (non-serious) with onset 07Dec2021 08:00, outcome "recovered" (Dec2021), described as "excruciating pain in both extremities below the knees"; MUSCULAR WEAKNESS (non-serious) with onset 07Dec2021 08:00, outcome "recovered" (Dec2021), described as "I noticed weakness in both lower extremities"; DYSSTASIA (non-serious) with onset 07Dec2021 08:00, outcome "recovered" (Dec2021), described as "I was unable to rise from a sitting position in a chair". The events "severe cramping in his legs, from the knees down", "woke up for about 50 minutes in the worst pain of his entire life", "when he looked at the muscles, they were rock solid, hard", "weakness in legs" were evaluated at the emergency room visit. The events "excruciating pain in both extremities below the knees", "i noticed weakness in both lower extremities" and "i was unable to rise from a sitting position in a chair" were evaluated at the physician office visit and emergency room visit. Therapeutic measures were taken as a result of muscle spasms, pain, muscle disorder, muscular weakness. Therapeutic measures were not taken as a result of arthralgia, muscular weakness, dysstasia.  Additional information: The patient had been taking Aspirin at least 4 or 5 years. The patient had no cerebrovascular disease, even though he never had a disease. The patient had no definite diagnosis of Alzheimer's. The patient did not receive other vaccine in four weeks. The patient did not have covid prior vaccination and had not been tested for covid post vaccination.  No follow-up attempts are possible. No further information is expected.