Do painkillers slow muscle growth from training?

Short answer

No. Taking 400 mg of ibuprofen after every workout for 6 weeks did not change muscle growth. But in a study where people took the maximum dose of 1,200 mg a day for 8 straight weeks, muscles grew only half as much.

Do painkillers slow muscle growth from training?
'Painkiller → less inflammation → weaker growth signal' — does this formula hold?
Muscle grows through the inflammation that follows a workout, so the story goes that anti-inflammatory painkillers blunt your gains. We checked studies where people trained while taking painkillers like ibuprofen.
The response right after training was blunted
Muscle-building speed did not rise
24 people did a hard leg workout, then took a maximum dose of painkiller (ibuprofen 1,200 mg a day or acetaminophen 4,000 mg a day) or a fake pill. 24 hours later, muscle-building speed was up 76% with the fake pill, but did not rise meaningfully with either painkiller.
But what about weeks of training?
At a normal dose, muscle grew the same
18 people trained their arms for 6 weeks, taking 400 mg of ibuprofen after training one arm and a fake pill after training the other. Arm muscle thickness rose from 3.63 to 3.92 cm and from 3.62 to 3.90 cm — the same — and strength did not differ either.
A maximum dose every day was different
Muscle grew only half as much in 8 weeks
31 people trained their legs for 8 weeks while taking either ibuprofen 1,200 mg or low-dose aspirin 75 mg every day. Thigh muscle grew 7.5% with aspirin and 3.7% with ibuprofen. Strength also rose less with ibuprofen in some exercises.
Flip side: in two 12-week studies, the painkiller group grew more
More muscle, but no extra strength
In a 12-week study of 36 people in their 60s, thigh muscle grew 8.6% with a fake pill, 10.9% with ibuprofen and 12.5% with acetaminophen. In 17 people taking diclofenac 75 mg for 12 weeks, muscle also grew more with the drug, 8.6% vs 3.9% — but strength did not rise more.
They did not speed up recovery either
Strength and muscle-damage markers stayed the same
An analysis pooling studies of strength training on painkillers found strength right after and 24 hours after training was the same as with a fake pill. Muscle-damage markers did not drop. In the 6-week study above, soreness was also the same in both arms.
Bottom line: an occasional painkiller does not block muscle
The issue is a maximum dose every day for weeks
No study found that a normal dose of painkiller made muscle grow less. Growth was cut in half only when people took ibuprofen 1,200 mg a day (6 tablets of 200 mg) every day for 8 weeks. Painkillers also did not speed up recovery from soreness, so there is no reason to take them out of habit because of training.