The takeaway
Amphastar P shows a pronounced seasonal pattern over 10 years of data — strongest in May (+6.3%) and softest in September (−3.8%).
Right now
In July, the stock has risen 50% of years, averaging +1.0%, roughly 1.2 pts behind the S&P 500.
The full picture
Amphastar P's most dependable month has been May, higher in 8 of 10 years; September has been its least reliable, up just 20% of the time.
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Month by month
The stock's clearest edge over the S&P 500 lands in May (+5.6 pts); it has trailed the market most in February (−4.1 pts).
“vs S&P” is Amphastar P’s average for a month minus the S&P 500’s average for that same month — isolating Amphastar P’s own seasonal edge from broad market drift.
Reality check
Over the last 5 years, May has closed higher 100% of the time versus 80% across the last 10 years — the pattern is strengthening.
Figures are the typical (median) May return and how often it rose — the last 5 years versus the last 10(the heatmap’s default window). This verdict stays anchored to that 10-year window even if you zoom the chart, so it never disagrees with the badges above.
In plain English
The seasonal story is really one month's story — May. It has closed higher in 8 of 10 Mays, a concentration the rest of the calendar can't touch.
The strength looks broad-based rather than freakish: its average (+6.3%) and median (+7.9%) sit close together, so no single blow-out year is flattering the figure. That reliability comes with real swings, mind — even May ranges by 12.6% from year to year, so any single year can land far from the average. Better still, that strength is the stock's own and not just a buoyant market — May has outpaced the S&P 500 by +5.6 points on average. Few peers keep such company in May — the typical stock clears it just 55% of the time.
It doesn't stand entirely alone — March, June, and November have leaned firm as well, if less emphatically. At the other end of the calendar, September has been the soft spot — the weakest of 5 months that average a loss (−3.8%), and the edge isn't year-round — the stock has trailed the S&P 500 in February, September, and January. Its roughest month on record was a −20.0% August in 2022 — a reminder of how hard even a seasonal name can fall.
May has now closed higher 6 years running. If anything it has sharpened recently — the last five Mays run ahead of the earlier years.
For a stock this dependable in May, the sharper question is the rest of the year — outside its strong stretch, the calendar gives far less to lean on.
Short answers on the stock's best month (May), its worst (September), and whether it really trades seasonally.
Yes, to a pronounced degree. Since 2016 its best month (May, +6.3%) has run well ahead of its worst (September, −3.8%) — the heatmap above shows how steady that gap has been year to year.
May has been the strongest, averaging +6.3% and closing higher in 8 of 10 years since 2016.
It's the weakest, averaging −3.8% — historically a soft spot, though it still varies from year to year.
Explore
These names have the strongest July track records on record — a starting point for comparison.
Before you trade